banlist: log post-swept banlist size at startup

Summary:
We are currently logging the size of the banlist before SweepBanned()
has been called, meaning the value may be incorrect.

This is a backport of core#19470

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9583