p2p: make gtxid(.hash) and fAlreadyHave localvars const

Summary:
Partial backport of core#19610:
https://github.com/bitcoin/bitcoin/pull/19610/commits/24ee4f01eadb870435712950a1364cf0def06e9f

There is very few remaining when removed the segwit content.

Depends on D9506.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9507