Add a clearByPrefix() method to the flood API

Rebased but didn't touch anything else, which (probably?) leaves me eligible to RTBC.

- Code changes make sense to me.
- All remarks made seem to be addressed.
- Green testbot

RTBC for me.

This needs a change record for the new interface.

Draft CR added.

Looks good now, hopefully the '-' prefix limitation will help Redis et al implement support for this without too much pain.

Committed be7d65e and pushed to 10.1.x. Thanks!

Seems we forgot to publish CR. Published it.

Automatically closed - issue fixed for 2 weeks with no activity.