Remove old unix sockets.
Json optimization nit.
Simplify handshake processing.
Rename connection_uid -> channel_uid.
Use tamer::at_preblock to reliably flush logs.
Keep up to 2 channels per peer replica.

In case the old channel has died. And send a message over the channel,
too, to detect its death.
Bug fix: Broadcast view to both old & new view members.
Bug fix: Send views more often.

Failed optimizations wouldn't send a view to a peer when:
- The peer wasn't in the new view (but it was in the old).
- The peer initiated a view change (reordering broadcast_view()
  and prepare() calls take care of that).

Also just send the view more often anyway.