When instantiating a Collection/IndexCatalog from a durable catalog entry it may contain a collection or index in drop-pending state. We need to be able to recover the instance.

The CollectionCatalog should contain a mapping from Ident to weak_ptr of these instances.
It needs to be a weak_ptr to not affect the lifetime and delay the actual drop from the reaper.

DDL operations putting collections/indexes in drop-pending should insert into this mapping and the cleanup should happen from the ident reaper when the drop has been performed.