Add an RPC to finalize a block

Summary:
A finalized block cannot be reorged. It effectively behaves very similarly to a checkpoint.

Based on work from @dagurval

Depends on D2057

Test Plan:

./test/functional/test_runner.py abc-finalize-block

Reviewers: #bitcoin_abc, schancel, jasonbcox

Reviewed By: #bitcoin_abc, schancel, jasonbcox

Subscribers: sickpig, dagurval, jasonbcox, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2083