Update Scheduler to not persist SchedulingPlan and accept equivalent placements

[tillrohrmann]

Right now the Scheduler persists its SchedulingPlan to the metadata store as a way to synchronize on it (in case there are multiple CCs running). With #2238, this is no longer strictly required. Especially, we add support to the Scheduler to also accept equivalent placement decisions that were originally taken by a different cluster controller. That way it is not tragic if there is a short period where multiple CCs believe that they are the leader.