Split the cs_main lock in SendMessages

Summary:
The PeerManager::SendMessages() method locks the cs_main for almost
the whole function. This diff splits the lock into pieces, slightly
reducing the scope and making it possible to later add avalanche parts
that don't lock cs_main.

Test Plan:
With clang and debug:

ninja all check-all
./test/functional/test_runner.py p2p_tx_download # extended

Run the TSAN build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9558