Clarify that m_tx_relay and m_proof_relay are thread safe

Summary: They CNode members are set once at construction time and then read only, but this is unclear because they are not declared const. This diff makes them const so this is clear they are thread safe.

Test Plan:

ninja all check-all

../contrib/teamcity/build-configurations.py build-tsan

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11556