banman: Add, use CBanEntry ctor that takes ban reason

Summary:
This is a partial backport of Core PR14605 : https://github.com/bitcoin/bitcoin/pull/14605/commits/c2e04d37f3841d109c1fe60693f9622e2836cc29

Depends on D4128

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4129