tx pool: Use class methods to hide raw map iterator impl details

Summary:
This is a backport of Core PR13793

Depends on D4599

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4601