Let a teardown's cancellation through, and reap the workers it leaves

Node stop paths cancelled their background task and then caught
CancelledError around the await, which swallows a cancellation aimed at
the caller — the trap Supervisor._cancel already documents. One shared
Node._cancel_task now waits the way the supervisor does; mqtt's publisher
and subscription and delay's cron call it.

The api container also collected zombie python workers: orphaned when
--reload replaces the process holding their handle, they reparent onto a
PID 1 that reaps nothing but its own. `init: true` on the backend service.

backend/fluksio/flow/nodes/delay.py

@@ -211,11 +211,7 @@ class DelayNode(Node):
         self._stop_cron.set()
 
         if self._cron_task is not None:
-            self._cron_task.cancel()
-            try:
-                await self._cron_task
-            except asyncio.CancelledError:
-                pass
+            await self._cancel_task(self._cron_task)
 
         self._cron_task = None
         self._stop_cron = None