Fix a potential issue with race condition when reloading a config

When SIGHUP is sent during load_config it's possible that
`config_reload_pending` will be set to `False` and new reload won't happen.

pglookout/pglookout.py

@@ -69,7 +69,7 @@ def __init__(self, config_path):
         self._config_version_applied = 0
         self._failover_on_disconnect = True
         self.load_config()
-        self.config_reload_pending = False
+        self.config_reload_pending = Queue()
 
         signal.signal(signal.SIGHUP, self.sighup)
         signal.signal(signal.SIGINT, self.quit)
@@ -113,7 +113,7 @@ def sighup(self, _signal=None, _frame=None):
             _signal,
             _frame,
         )
-        self.config_reload_pending = True
+        self.config_reload_pending.put(True)
 
     def load_config(self):
         self.log.debug("Loading JSON config from: %r", self.config_path)
@@ -858,9 +858,9 @@ def _get_check_interval(self) -> float:
 
     def main_loop(self):
         while self.running:
-            if self.config_reload_pending:
+            while not self.config_reload_pending.empty():
                 self.load_config()
-                self.config_reload_pending = False
+                self.config_reload_pending.get()
             try:
                 self._apply_latest_config_version()
             except Exception as ex:  # pylint: disable=broad-except