banman: create and split out banman

Summary:
Some say he has always been.

This is a partial backport of Core PR14605 : https://github.com/bitcoin/bitcoin/pull/14605/commits/4c0d961eb0d7825a1e6f8389d7f5545114ee18c6

Depends on D4091

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4114