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.

docker/compose.yml

@@ -75,6 +75,13 @@ services:
     image: '${DOCKER_IMAGE_BACKEND?Variable not set}:${TAG-latest}'
     container_name: fluksio-api
     restart: always
+    # PID 1 that reaps orphans. A python node's worker processes outlive the
+    # process holding their handle whenever that one goes without stopping the
+    # pool -- which `--reload` does on every source edit -- and reparent onto
+    # PID 1, which is the app itself and waits for nobody else's children. The
+    # container filled up with zombie `python`. Declared here rather than in
+    # compose.dev.yml because an init closes the whole class, not just reload.
+    init: true
     security_opt:
       - no-new-privileges:true
     networks: