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.
Keep clicking, you might help!

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/tasks/signals.py

@@ -9,12 +9,17 @@ logger = logging.getLogger('awx.main.tasks.signals')
 __all__ = ['with_signal_handling', 'signal_callback']
 
 
+class SignalExit(Exception):
+    pass
+
+
 class SignalState:
     def reset(self):
         self.sigterm_flag = False
         self.is_active = False
         self.original_sigterm = None
         self.original_sigint = None
+        self.raise_exception = False
 
     def __init__(self):
         self.reset()
@@ -22,6 +27,9 @@ class SignalState:
     def set_flag(self, *args):
         """Method to pass into the python signal.signal method to receive signals"""
         self.sigterm_flag = True
+        if self.raise_exception:
+            self.raise_exception = False  # so it is not raised a second time in error handling
+            raise SignalExit()
 
     def connect_signals(self):
         self.original_sigterm = signal.getsignal(signal.SIGTERM)