Move server startup logging

This way, another log output is printed when the server is restarted by
a supervisor.
Supervise the server
Restart whole bridge in case of a crash

This is not the most elegant or defensive solution, but by linking all actors
to the server, we make sure that a crashing actor will cause the whole bridge
to restart.

Clients connected via WebSocket will notice the dropped connection and try to
reconnect again.
Add basic command parameter validation