This commit adds a Sync message which is sent as a reply to Connect
message. This should in theory speed up convergence of a (re)connecting
node.
We respond to Sync message by sending a peer message back to the peer
origin of the Sync message. We consequently update our routing table and
peer graph with the data sent in via Sync message.
We now gossip advertse to up to 3 randomly selected peers instead of
sending a multicast message to the network.
Equally, Solicitation i.e. full table adverts are gossipped to a
randomly selected peer. If that fails we send a multicast message to the
network.
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