reduce parent->child lock contention

* We update the parent unified job template to point at new jobs
created. We also update a similar foreign key when the job finishes
running. This causes lock contention when the job template is
allow_simultaneous and there are a lot of jobs from that job template
running in parallel. I've seen as bad as 5 minutes waiting for the lock
when a job finishes.
* This change moves the parent->child update to OUTSIDE of the
transaction if the job is allow_simultaneous (inherited from the parent
unified job). We sacrafice a bit of correctness for performance. The
logic is, if you are launching 1,000 parallel jobs do you really care
that the job template contains a pointer to the last one you launched?
Probably not. If you do, you can always query jobs related to the job
template sorted by created time.

awx/main/models/unified_jobs.py

@@ -873,7 +873,13 @@ def save(self, *args, **kwargs):
 
         # If status changed, update the parent instance.
         if self.status != status_before:
-            self._update_parent_instance()
+            # Update parent outside of the transaction for Job w/ allow_simultaneous=True
+            # This dodges lock contention at the expense of the foreign key not being
+            # completely correct.
+            if getattr(self, 'allow_simultaneous', False):
+                connection.on_commit(self._update_parent_instance)
+            else:
+                self._update_parent_instance()
 
         # Done.
         return result