Require no cs_main lock for ProcessNewBlock/ActivateBestChain

Summary:
This requires the removal of some very liberal (incorrect) cs_mains
sprinkled in some tests. It adds some chainActive.Tip() races, but
the tests are all single-threaded anyway.

Depends on D2607, D2617 and D2631

Partial backport of core PR11824 (commit a99b76f)

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2608