Add CallFunctionInQueue to wait on validation interface queue drain

Summary:
This is extracted from Core's PR10286 commit 0b2f42d7376c5f7c1ba1ac5d17a30691989d9159

Depends on D2265

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2266