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.