Merge #13116: Add Clang thread safety annotations for variables guarded by cs_{rpcWarmup,nTimeOffset,warnings}

Summary:
8499f15e67 Add Clang thread safety annotations for variables guarded by cs_warnings (practicalswift)
cf13ad23d4 Add Clang thread safety annotations for variables guarded by cs_nTimeOffset (practicalswift)
012dec0347 Add Clang thread safety annotations for variables guarded by cs_rpcWarmup (practicalswift)

Pull request description:

Add Clang thread safety annotations for variables guarded by `cs_{rpcWarmup,nTimeOffset,warnings}`.

Tree-SHA512: 8e0a4b9e36a4450bd75ad32c21d813bb572aaaa5b4a4cbdcbf4678e58ade6265c0b275352391168930a63fcbd09caa3b76e74595a7b14646054c52870c46d007

Backport of Core PR13116

Test Plan:

ninja

Run build-werror configuration

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6317