Auto-finalize block once they reached a certain depth (by default 10)

Summary:
This ensures that deep reorg are not possible.

Can be controlled by the -maxreorgdepth parameter. Set to -1 to disable.

Depends on D2083

Test Plan: Added an integration test.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2102