Remove transport protocol knowhow from CNetMessage / net processing

Summary:
Partial backport 2/7 of core PR16202:
https://github.com/bitcoin/bitcoin/pull/16202/commits/1a5c656c3169ba525f84145d19ce8c64f2cf1efb

Depends on D8222.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, jasonbcox

Reviewed By: #bitcoin_abc, PiRK, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8223