Implement BetterHash Mining Protocol

[lehnberg]

trafficstars

Documentation
BIP	https://github.com/TheBlueMatt/bips/blob/betterhash/bip-XXXX.mediawiki
Meeting notes	https://github.com/mimblewimble/grin-pm/blob/master/notes/20181016-meeting-development.md#4-betterhash-mining-protocol
Forum thread	https://www.grin-forum.org/t/betterhash-mining-protocol/913

Motivation

Improves on the currently implemented Stratum protocol while also separating block construction from payouts. While this continues to allow miners to pool payouts to reduce variance, they will also be able to select which transactions to mine and signal their readiness for protocol upgrades. Thus improving the degree of decentralisation in mining pools.

[mcdallas]

This seems like a good place to start https://github.com/TheBlueMatt/mining-proxy . It's a rust implementation by the author