Move fill_mempool to util function

Summary:
This is a partial backport of core#29735, consisting of the first 3 commits.

The remaining parts of the PR fix a bug that has not been introduced (dependency not backported yet).

Test Plan:

./test/functional/test_runner.py mempool_limit

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16486