Misbehavior: Only slash misbehaving validators that were opted-in recently

## Problem

Ideally, we would like to slash a validator on a consumer chain `C` only if validator `C` had opted-in during the last `UnbondingPeriod` and **not** slash if the validator was opted in much earlier than that but has since opted-out (i.e., the validator was never opted in during the last `UnbondingPeriod`). Search `TODO (#2242)` in the codebase on where this could help.