[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.
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2259