The entity map needs to be able to distinguish between the case when
the entity_id is waiting to be dropped, and when it is completely gone.

Before 8bc207141, it assumed that entity_ids in dropped_entity_ids could
be re-used. This caused `take_dropped` to error because the slot had
been overwritten. The fix there caused weak handles to allow upgrading
a reference count from 0, which could resurrect items in
`dropped_entity_ids` which caused them to be dropped twice.

We could allow weak items to upgrade from 0, and delete from
dropped_entity_ids, but that seemed more complicated than necessary.