better peer management


This remove the old 'copayers' message type in the protocol to broadcast the connected copayers.

With the new schema we have been using for 2 weeks, that was unneeded, each peer can connect and reconnect based on the pubkey ring.  In this pull also the authentication and mapping of the peers have been enhanced and simplified. 

Also, a basic reconnect schema is used: after any network event, each peer try to reconnect to the unconnected peers in the ring.

Fixes https://github.com/bitpay/copay/issues/193