Send events on Manager startup & shutdown. To make this possible, events
sent to MQTT are now queued up until an MQTT server can be reached.
Otherwise the startup event would be sent before the MQTT connection was
established.
Fix SocketIO subscriptions so that the client also subscribes to
job-specific last-rendered images whenever subscribing to job-specific
events. These are sent to another event topic, and thus need some extra
care. Before the introduction of the generic event bus, both message types
were sent to the same topic, but that's not supported by MQTT, and so things
had to change.
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.
