Make the sequencer more solid

[asonnino]

This PR makes the sequencer more robust so that we can use it for dev net. It remains a testing facility, but it should not make our life miserable when deploying/re-deploying networks of authorities.

Particularly:

• The sequencer is now crash-safe
• The core sequencer does not get stuck if one of the subscribers is (unnaturally) slow
• The core sequencer is memory-bound.

Note that the sequencer cannot re-use the store of the authority since we have a single sequencer for all authorities (thus running on a single machine).