Refactor: split network transport deserializing from message container

Summary:
Partial backport 1/7 of core PR16202:
https://github.com/bitcoin/bitcoin/pull/16202/commits/6294ecdb8bb4eb7049a18c721ee8cb4a53d80a06

Adapted to match our codebase.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, jasonbcox

Reviewed By: #bitcoin_abc, PiRK, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8222