Record the real BEAM block hash instead of pow hash to the database

Beam poolwatcher: If beam-node supports it, write the real block hash instead of pow hash to the database.

Use this beam node to response the real block hash to poolwatcher:
https://github.com/btccom/beam/tree/mining-btccom
Or docker:
https://github.com/btccom/btcpool/tree/master/docker/beam-node/main-release-mining