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

[insumity]

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.