net: move BanMan to its own files

Summary:
This is a partial backport of Core PR14605 : https://github.com/bitcoin/bitcoin/pull/14605/commits/af3503d903b1a608cd212e2d74b274103199078c

Depends on D4124

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4125