[RCU] Interlock synchronize operation when there is contention

Summary: In order to avoid starving the computer's reources, we make sure that the synchronize operation that last for long execute one by one using a mutex.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2259