With the per-broadcast-source GossipSenders gone, we can get rid of
GossipSender gc...

Instead of maintaining a Connection->GossipSender map in the
GossipChannels - which we then need to gc when connections get closed
- we keep gossip_channel_name->GossipSender maps in LocalConnections.

Access to the map is guarded by a separate lock, rather than re-using
the LocalConnection lock. This reduces lock contention and risk of
deadlock.

In the GossipSenders, we replace the Stop() method with a signalling
channel, i.e. senders stop when that channel gets closed. Handily, the
LocalConnection.finished channel fits that description. So when a
LocalConnection's run loop terminates, any existing and future
associated GossipSenders will end up terminating too.