Create new mutex for orphans, no cs_main in PLV::BlockConnected

Summary:
This should (marginally) speed up validationinterface queue
draining by avoiding a cs_main lock in one client.

Partial backport of core PR11824 (commit 818075a)

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/D2605