Prevent auto-finalization from moving backward the finalization point

Summary:
This fix an issue where the auto-finalization behaviour could cause
unexpected rewind of the finalization chain by an amount of
maxreorgdepth blocks.

Depends on D2130

Test Plan:

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

Reviewers: deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2161