p2p, refactor: add CInv transaction message helpers; use in net processing

Summary:
Our code differs from core on this part. In order to kept the changes
minimal and limit conflicts I kept our implementation but renamed the
function to match core.

Backport of core#19590.

Test Plan:

ninja all check-all
ninja bitcoin-fuzzers

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9500