Mark CTxMemPoolEntry members that should not be modified const

Summary: This is a partial backport of Core PR13792 : https://github.com/bitcoin/bitcoin/pull/13792/commits/ddd395f968a050be5dd0ae21ba7d189b6b7f73fd

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4914