Introduce an "event bus"-like system. It's more like a fan-out
broadcaster for certain events. Instead of directly sending events to
SocketIO, they are now sent to the broker, which in turn sends it to any
registered "forwarder". Currently there is ony one forwarder, for
SocketIO.
This opens the door for a proper MQTT client that sends the same events
to an MQTT server.
Change the package base name of the Go code, from
`git.blender.org/flamenco` to `projects.blender.org/studio/flamenco`.
The old location, `git.blender.org`, has no longer been use since the
[migration to Gitea][1]. The new package names now reflect the actual
location where Flamenco is hosted.
[1]: https://code.blender.org/2023/02/new-blender-development-infrastructure/
Workers can be soft-deleted, which means that they stay in the database.
As such, foreign key constraints `ON DELETE CASCADE` do not trigger, and
thus their sleep schedule can still be active. This is now detected and
handled gracefully.
When the Manager was shutting down while the sleep scheduler was running, it
could cause a null pointer dereference. This is now doubly solved:
- `worker.Identifier()` is now nil-safe, as in, `worker` can be `nil` and
it will still return a sensible string.
- failure to apply the sleep schedule due to the context closing is not
logged as error any more.
