Realm: Create new deferred object when destroying sparsity map

We need to be able to support multiple deferred destroy operations on the same sparsity map. EventImpl::add_waiter(wait_on, deferred_object) "push_backs" an object intro intrusive list. We can only use one of this object in a list at a time which breaks our use-case for deferred ref-counting. This change makes us go through the new path on each deferred destroy call much like deferred reservation.acquire/release are done today.

Tested on computelab - hang does not occur