Add LeaderBoard API endpoint #6
Description
API endpoint that provide a list of reachable nodes ranked according to their Peer Index (PIX) as of the latest snapshot retrieved. PIX values are calculated only for activated nodes.
Peer Index (PIX) is defined ad described in this document:
https://bitnodes.earn.com/nodes/leaderboard/#peer-index
This is needed to re-enable a source of seeding BUCash was using, see BitcoinUnlimited/BitcoinUnlimited#928 for more details