Merge #15697: qa: Make swap_magic_bytes in p2p_invalid_messages atomic

Summary:
faca95effd qa: Make swap_magic_bytes in p2p_invalid_messages atomic (MarcoFalke)

Pull request description:

Otherwise, this will lead to errors logged in the network thread:

https://travis-ci.org/MarcoFalke/bitcoin/jobs/513076282#L2765

ACKs for commit faca95:

Tree-SHA512: 8bc8280f9c0e8d40bc68c0ff1cab1c3386cbfef0728dbab5f29c2606dd65a9ac3c695b0c286be707a9f2bd62a6f87ac2032d7749fc2cf8b9fa1eba3c4cf70933

Backport of Core PR15697

Depends on D6395

Test Plan: test_runner.py p2p_invalid_messages

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6396