Add protocol to handshake between nodes
There is a need to be able to extend the protocol without breaking compatibility with old nodes. This is done by sending version.protocol during inter-node handshake, in seqno 1 and seqno 2, so that a node knows what version the peers speak and use appropriate format. This is implemented with partial backward compatibility: handshake with an old node succeeds when the new node does not have to send seqno 1.
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