p2p: use CInv block message helpers in net_processing.cpp

Summary:
Partial backport of core#19610:
https://github.com/bitcoin/bitcoin/pull/19610/commits/b1c855453bf2634e7fd9b53c4a76a8536fc9865d

Depends on D9505.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9506