Move peer state updates from headers message into separate function

Summary:
Review hint:
git show --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

This is a partial backport of core#25454
https://github.com/bitcoin/bitcoin/pull/25454/commits/6d95cd3e7444ebaaabb64a76783ea3551530f1d7

Depends on D14909

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14910