Fix memory leak and reduce overall memory usage

[pm47]

This PR adds better management of resources, mainly:

• define a proper supervisor strategy in the authenticator
• sql PreparedStatement are now explicitely closed
• the origins map (which allows to not ping-pong announcements to the same peer) has been removed, instead the stash/rebroadcast/awaiting collections all maintain a reference to the origin peer

When a channel enters the CLOSING state:

• we remember which transactions have already reached min_depth, and don't republish/watch them on restart
• we don't watch the funding tx as soon as we know that it has been spent by a tx which has reached min_depth

Also, the zmq watcher doesn't trigger a TickNewBlock on new WatchConfirmed anymore, this was causing a massive herd effect on startup.

Note that this PR implies a non-backward-compatible change in DATA_CLOSING.

Fixes #357.