Add master pseudo-component

The master component launches and supervises the components specified in
the configuration file.

It is a pseudo-component as it is not registered in pythia.Components.
Instead, it is used when pythia is started with no specified component.
Improve logging
backend/queue: Fix client disconnect

When a client disconnects, it shall be removed from the queue.clients
map. Otherwise, the response channel will be closed twice (and thus
panic).