sync.h: add REVERSE_LOCK

Summary:
This is a partial backport of Core PR18234 : https://github.com/bitcoin/bitcoin/pull/18234/commits/b9c426012770d166e6ebfab27689be44e6e89aa5

Depends on D6531

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6532