Fix/stabilizing-observers

[einari]

Fixed

• Adding fail safe error handling for ConnectionService during keep alive. If it fails within the keep alive loop, it will keep on going and not just die. Added logging if this occurs.
• Added more error handling for AppendedEventsQueue to ensure the queue task is always alive, and if it stops it will be recreated. Logging the errors if they occur.
• If a call to Handle() on the Observer fails, the AppendedEventsQueue will catch this and retry indefinitely. Any failures happening within the Observer should be handled by itself and rais failed partitions. But not being able to reach the Observer should not mean we move on to the next event. This might need a circuit breaker. For now we consider this scenario as a transient failure.
• Observers will now not handle any events unless it is in the Active state. This is to prevent handling happening while transitioning between states.
• Projections clean up after themselves by unregistering for definition changes.