Implement support for ephemeral headers

[acolytec3]

Internal work related to support for ephemeral headers - partially fixes #741

• Implement support for storing/retrieving ephemeral headers in the HistoryNetwork DB
• Maintain an ephemeralHeaderIndex of block numbers to hashes - this is stored in memory for ready access to the headers (which are only stored by blockhash in the DB)
• Report the current number of ephemeral headers contained in our DB in the History_Radius ping/pong payloads
  • Add pingPongPayload to HistoryNetwork - added the History Network specific payload extension here
  • Remove History_Radius from BaseNetwork - Removed from base network as it doesn't make sense to have specific subnetwork stuff in our BaseNetwork class

[ScottyPoi]

LGTM