Move cs_main locking annotations from .cpp to .h

Summary:
Move cs_main locking annotations from .cpp to .h.

Backport of Bitcoin Core PR14063
https://github.com/bitcoin/bitcoin/pull/14063

Test Plan:
Maybe not necessary because are "aesthetic changes", but just in case:

1. Build with Clang in Debug mode:

CXX=clang++ CC=clang cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug
ninja check

2. Verify that the compiler has not emitted a thread-safety warning.
3. Run the node: ./src/bitcoind -regtest
4. Verify that text similar to "Assertion failed: lock ... not held ..." is not printed on stderr.

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4149