[healthd] fix healthd shutdown race (sonic-net#19504)

#### Why I did it

To fix errors that happen when writing to the queue:

```
Jun  5 23:04:41.798613 r-leopard-56 NOTICE healthd: Caught SIGTERM - exiting...
Jun  5 23:04:41.798985 r-leopard-56 NOTICE healthd: Caught SIGTERM - exiting...
Jun  5 23:04:41.799535 r-leopard-56 NOTICE healthd: Caught SIGTERM - exiting...
Jun  5 23:04:41.806010 r-leopard-56 NOTICE healthd: Caught SIGTERM - exiting...
Jun  5 23:04:41.814075 r-leopard-56 ERR healthd: system_service[Errno 104] Connection reset by peer
Jun  5 23:04:41.824135 r-leopard-56 ERR healthd: Traceback (most recent call last):#12  File "/usr/local/lib/python3.9/dist-packages/health_checker/sysmonitor.py", line 484, in system_service#012    msg = self.myQ.get(timeout=QUEUE_TIMEOUT)#12  File "<string>", line 2, in get#012  File "/usr/lib/python3.9/multiprocessing/managers.py", line 809, in _callmethod#012    kind, result = conn.recv()#12  File "/usr/lib/python3.9/multiprocessing/connection.py", line 255, in recv#012    buf = self._recv_bytes()#12  File "/usr/lib/python3.9/multiprocessing/connection.py", line 419, in _recv_bytes#012    buf = self._recv(4)#12  File "/usr/lib/python3.9/multiprocessing/connection.py", line 384, in _recv#012    chunk = read(handle, remaining)#012ConnectionResetError: [Errno 104] Connection reset by peer
Jun  5 23:04:41.826489 r-leopard-56 INFO healthd[8494]: ERROR:dbus.connection:Exception in handler for D-Bus signal:
Jun  5 23:04:41.826591 r-leopard-56 INFO healthd[8494]: Traceback (most recent call last):
Jun  5 23:04:41.826640 r-leopard-56 INFO healthd[8494]:   File "/usr/lib/python3/dist-packages/dbus/connection.py", line 232, in maybe_handle_message
Jun  5 23:04:41.826686 r-leopard-56 INFO healthd[8494]:     self._handler(*args, **kwargs)
Jun  5 23:04:41.826738 r-leopard-56 INFO healthd[8494]:   File "/usr/local/lib/python3.9/dist-packages/health_checker/sysmonitor.py", line 82, in on_job_removed
Jun  5 23:04:41.826785 r-leopard-56 INFO healthd[8494]:     self.task_notify(msg)
Jun  5 23:04:41.826831 r-leopard-56 INFO healthd[8494]:   File "/usr/local/lib/python3.9/dist-packages/health_checker/sysmonitor.py", line 110, in task_notify
Jun  5 23:04:41.826877 r-leopard-56 INFO healthd[8494]:     self.task_queue.put(msg)
Jun  5 23:04:41.826923 r-leopard-56 INFO healthd[8494]:   File "<string>", line 2, in put
Jun  5 23:04:41.826973 r-leopard-56 INFO healthd[8494]:   File "/usr/lib/python3.9/multiprocessing/managers.py", line 808, in _callmethod
Jun  5 23:04:41.827018 r-leopard-56 INFO healthd[8494]:     conn.send((self._id, methodname, args, kwds))
Jun  5 23:04:41.827065 r-leopard-56 INFO healthd[8494]:   File "/usr/lib/python3.9/multiprocessing/connection.py", line 211, in send
Jun  5 23:04:41.827115 r-leopard-56 INFO healthd[8494]:     self._send_bytes(_ForkingPickler.dumps(obj))
Jun  5 23:04:41.827158 r-leopard-56 INFO healthd[8494]:   File "/usr/lib/python3.9/multiprocessing/connection.py", line 416, in _send_bytes
Jun  5 23:04:41.827199 r-leopard-56 INFO healthd[8494]:     self._send(header + buf)
Jun  5 23:04:41.827254 r-leopard-56 INFO healthd[8494]:   File "/usr/lib/python3.9/multiprocessing/connection.py", line 373, in _send
Jun  5 23:04:41.827322 r-leopard-56 INFO healthd[8494]:     n = write(self._handle, buf)
Jun  5 23:04:41.827368 r-leopard-56 INFO healthd[8494]: BrokenPipeError: [Errno 32] Broken pipe
Jun  5 23:04:42.800216 r-leopard-56 NOTICE healthd: Caught SIGTERM - exiting...
```

When the multiprocessing.Manager is shutdown the queue will raise the above errors. This happens during shutdown - fast-reboot, warm-reboot.


With the fix, system-health service does not hang:

```
root@sonic:/home/admin# sudo systemctl start system-health ; sleep 10; echo "$(date): Stopping..."; sudo systemctl stop system-health; echo "$(date): Stopped"
Thu Oct 17 01:07:56 PM IDT 2024: Stopping...
Thu Oct 17 01:07:58 PM IDT 2024: Stopped
root@sonic:/home/admin# sudo systemctl start system-health ; sleep 10; echo "$(date): Stopping..."; sudo systemctl stop system-health; echo "$(date): Stopped"
Thu Oct 17 01:08:13 PM IDT 2024: Stopping...
Thu Oct 17 01:08:14 PM IDT 2024: Stopped
root@sonic:/home/admin# sudo systemctl start system-health ; sleep 10; echo "$(date): Stopping..."; sudo systemctl stop system-health; echo "$(date): Stopped"
Thu Oct 17 01:09:05 PM IDT 2024: Stopping...
Thu Oct 17 01:09:06 PM IDT 2024: Stopped
```

##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it

Remove the call to shutdown, the cleanup will happen automatically when GC runs as per documentation - https://docs.python.org/3/library/multiprocessing.html

#### How to verify it

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Run warm-reboot, fast-reboot multiple times and verify no errors in the log.

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- [ ] 201911
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- [ ] 202106
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- [x] 202205
- [x] 202311
- [x] 202405

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src/system-health/health_checker/sysmonitor.py

@@ -104,6 +104,11 @@ def task_worker(self):
         logger.log_info("Start Listening to systemd bus (pid {0})".format(os.getpid()))
         self.subscribe_sysbus()
 
+    def task_stop(self):
+        # FIXME: Gracefully stop `loop.run()`.
+        self._task_process.kill()
+        return True
+
     def task_notify(self, msg):
         if self.task_stopping_event.is_set():
             return
@@ -481,9 +486,11 @@ def system_service(self):
 
         from queue import Empty
         # Queue to receive the STATEDB and Systemd state change event
-        while not self.task_stopping_event.is_set():
+        while True:
             try:
                 msg = self.myQ.get(timeout=QUEUE_TIMEOUT)
+                if msg == "stop":
+                    break
                 event = msg["unit"]
                 event_src = msg["evt_src"]
                 event_time = msg["time"]
@@ -503,28 +510,19 @@ def system_service(self):
         monitor_statedb_table.task_stop()
 
     def task_worker(self):
-        if self.task_stopping_event.is_set():
-            return
         self.system_service()
 
     def task_stop(self):
-        # Signal the process to stop
-        self.task_stopping_event.set()
-        #Clear the resources of mpmgr- Queue
-        self.mpmgr.shutdown()
+        self.myQ.put("stop")
 
         # Wait for the process to exit
         self._task_process.join(self._stop_timeout_secs)
 
         # If the process didn't exit, attempt to kill it
         if self._task_process.is_alive():
             logger.log_notice("Attempting to kill sysmon main process with pid {}".format(self._task_process.pid))
-            os.kill(self._task_process.pid, signal.SIGKILL)
-
-        if self._task_process.is_alive():
-            logger.log_error("Sysmon main process with pid {} could not be killed".format(self._task_process.pid))
+            self._task_process.kill()
+            self._task_process.join()
             return False
 
         return True
-
-