Partially revert core PR5927

Summary: This reintroduce code that check headers against known checkpoint. This ensures we do not accept a chain of header that is lacking the checkpoint we know of.

Test Plan:
Also reverted the test.
This would probably deserve an integration test in the QA suite, but considering the code has existed for a very long time, we can assume it works.

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1308