Block ActivateBestChain to empty validationinterface queue

Summary:
Partial backport of core PR11824 (commit 3613749)

Depends on D2609

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2610