[RCU] Use strong CAS where apropriate

Summary:
Failing the CAS in RCUInfos's destructor require to free and retake a lock, and go over a linked list. We should avoid doing so at it is more expensive that any penality a strong CAS have over a weak one.

The CAS in RCUInfos benefit from being weak as the loop is required anyways and is made of a single store.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2332