Refactor MQTT (#24010)

* refactor mqtt so that Frigate owns the transport lifecycle instead of delegating it to paho

* release the shutdown barrier on worker crash and replay retained publishes the broker never acked

* collapse in-flight retained values by topic and release the shutdown barrier from a finally

* replay the outage buffer before the publish queue so newer values are not reverted

frigate/comms/inter_process.py

@@ -18,10 +18,13 @@ SOCKET_REP_REQ = "ipc:///tmp/cache/comms"
 
 class InterProcessCommunicator(Communicator):
     def __init__(self) -> None:
+        # bound eagerly so subprocesses starting before start_communicators()
+        # can still connect; their requests queue in zmq until the reader runs
         self.context = zmq.Context()
         self.socket = self.context.socket(zmq.REP)
         self.socket.bind(SOCKET_REP_REQ)
         self.stop_event: MpEvent = mp.Event()
+        self.reader_thread: threading.Thread | None = None
 
     def publish(self, topic: str, payload: Any, retain: bool = False) -> None:
         """There is no communication back to the processes."""
@@ -29,6 +32,8 @@ class InterProcessCommunicator(Communicator):
 
     def subscribe(self, receiver: Callable) -> None:
         self._dispatcher = receiver
+
+    def start(self) -> None:
         self.reader_thread = threading.Thread(target=self.read)
         self.reader_thread.start()
 
@@ -61,7 +66,8 @@ class InterProcessCommunicator(Communicator):
 
     def stop(self) -> None:
         self.stop_event.set()
-        self.reader_thread.join()
+        if self.reader_thread is not None:
+            self.reader_thread.join()
         self.socket.close(linger=0)
         self.context.destroy(linger=0)