Add thread safety annotations to RNG state

Summary:
Partial backport of Core PR14955
https://github.com/bitcoin/bitcoin/pull/14955/commits/aae8b9bf0f4fd2b801ee72cf191588c8b3a67c3c

Depends on D4391

Test Plan:

contrib/teamcity/build-configurations.sh build-werror

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4392