switch the periodic scheduler to a child process (instead of a thread)

I have a hunch that our usage of a daemon thread is causing import lock
contention related to https://github.com/ansible/awx/issues/5617
We've encountered similar issues before with threads across dispatcher
processes at fork time, and cpython has had bugs like this in recent
history:

https://bugs.python.org/issue38884

My gut tells me this might be related.

The prior implementation - based on celerybeat - ran its code in
a process (not a thread), and the timing of that merge matches the
period of time we started noticing issues.

Currently testing it to see if it resolves some of the issues we're
seeing.

awx/main/management/commands/run_dispatcher.py

@@ -53,7 +53,7 @@ class Command(BaseCommand):
 
         # spawn a daemon thread to periodically enqueues scheduled tasks
         # (like the node heartbeat)
-        cease_continuous_run = periodic.run_continuously()
+        periodic.run_continuously()
 
         reaper.reap()
         consumer = None
@@ -87,7 +87,6 @@ class Command(BaseCommand):
                 )
                 consumer.run()
             except KeyboardInterrupt:
-                cease_continuous_run.set()
                 logger.debug('Terminating Task Dispatcher')
                 if consumer:
                     consumer.stop()