Implement miner funding features

Summary: This implements a whitelist, activated via BIP9. Each block must send 5% of the block reward to one of the activated addresses.

Test Plan: Add integration tests.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: Mengerian, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5282