Merge #11028: Avoid masking of difficulty adjustment errors by checkpoints

Summary:
85c82b5 Avoid masking of difficulty adjustment errors by checkpoints (Pieter Wuille)

Pull request description:

Currently difficulty adjustment violations are not reported for chains that branch off before the last checkpoint. Change this by moving the checkpoint check after the difficulty check.

Tree-SHA512: 33666f2c3459151b28c42041a463779e6df18f61d3dd5b1879a0af4e5b199ef74d1e33e06af68bebfdfb211569ad5fb56556bfebe9d63b5688d910ea211b839a

Backport of Core PR 11028
https://github.com/bitcoin/bitcoin/pull/11028/files
Depends on D2722

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2723