Serialize index creation through claimant

If index creation events are executed on disjoint nodes in a tight enough time span then the operations can create data races on the ring and false removal events can be generated by the `yz_events` delta logic.  Rather than try to get clever and allow concurrent operations instead serialize all index operations through the claimant.  This means that index creation/removal cannot be performed if the claimant is down, but avoids the data race.