Implement EBx indicator in coinbase message

Summary: Implement EBx indicator in coinbase message

Test Plan: src/test/test_bitcoin -t miner_tests

Reviewers: deadalnix, freetrader, kyuupichan, awemany, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D249