Refactor canceling to work through messaging and signals, not database

If canceled attempted before, still allow attempting another cancel
in this case, attempt to send the sigterm signal again.
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Replace other cancel_callbacks with sigterm watcher
  adapt special inventory mechanism for this too

Get rid of the cancel_watcher method with exception in main thread

Handle academic case of sigterm race condition

Process cancelation as control signal

Fully connect cancel method and run_dispatcher to control

Never transition workflows directly to canceled, add logs

awx/main/utils/handlers.py

@@ -76,7 +76,7 @@ class SpecialInventoryHandler(logging.Handler):
     def emit(self, record):
         # check cancel and timeout status regardless of log level
         this_time = now()
-        if (this_time - self.last_check).total_seconds() > 0.5:  # cancel callback is expensive
+        if (this_time - self.last_check).total_seconds() > 0.1:
             self.last_check = this_time
             if self.cancel_callback():
                 raise PostRunError('Inventory update has been canceled', status='canceled')