Add an interface to get the queue depth out of CValidationInterface

Summary:
Partial backport of core PR11824 (commit 5a933ce)
Depends on D2608

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2609