net: Add Clang thread safety annotations for guarded variables in the networking code

Summary:

• Add missing lock in CNode::copyStats(...)
• Add missing locking annotations

This is a backport of Core PR13123

Depends on D4083

Test Plan:

cmake -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ ..
ninja

Check that there are no thread safety analysis warnings.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4084