Fix concurrency-related bugs in ActivateBestChain

Summary:
If multiple threads are invoking ActivateBestChain, it was possible to have
them working towards different tips, and we could arrive at a less work tip
than we should. Fix this by introducing a ChainState lock which must
be held for the entire duration of ActivateBestChain to enforce
exclusion in ABC.

This is a partial backport of Core PR13023 : https://github.com/bitcoin/bitcoin/pull/13023/commits/a3ae8e68739023e5dba9e5cb190e707ed4603316

Depends on D3435

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3436