Avoid cs_main in net_processing ActivateBestChain calls

Summary:
Partial backport of core PR11824 (commit a734896)
Depends on D2606

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2607