banman: pass in default ban time as a parameter

Summary:
Removes the dependency on arg parsing.

This is a partial backport of Core PR14605 : https://github.com/bitcoin/bitcoin/pull/14605/commits/d0469b2e9386a7a4b268cb9725347e7517acace6

Depends on D4117

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4124