[refactor] return MempoolAcceptResult

Summary:
This creates a cleaner interface with ATMP, allows us to make results const,
and makes accessing values that don't make sense (e.g. fee when tx is
invalid) an error.

This is a backport of core#21062 [2/4]
https://github.com/bitcoin/bitcoin/pull/21062/commits/f82baf0762f60c2ca5ffc339b095f9271d7c2f33

Note: changes related to RBF are not applicable

Depends on D11778

Test Plan: ninja all check-all bitcoin-bench

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11779