banman: add thread annotations and mark members const where possible

Summary:
Also remove misleading comment. ClearBanned is used by rpc as well.

This is a partial backport of Core PR14605 : https://github.com/bitcoin/bitcoin/pull/14605/commits/daae598feb034f2f56e0b00ecfb4854d693d3641

Depends on D4126

Test Plan: Build with clang and check no warning is thrown

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4127