Improve net_plugin syncing via communication of earliest available block

[heifner]

Currently the net_plugin handshake message does not include earliest available block information which causes many requests from peers for blocks that a node does not have.

Include the earliest available block on a node in the handshake message. One potential way would be to look at the peers network_version after the initial handshake and send a handshake_v2 which includes the earliest block info. handshake_v2 could be detected according to network_version of the handshake to know if any additional information is included after the end of handshake_message. This additional information can't be sent until verification of the peer's version as an older version of net_plugin would not correctly handle the additional information after the handshake_message data.

[stephenpdeos]

This is work that may be made obsolete due to upcoming P2P work.

[heifner]

Subsumed by AntelopeIO/leap#1072