DeletedEntries always deallocates all blocks

[Veykril]

We currently use crossbeams SeqQueue for our DeletedEntries management (deferred dropping) which general seems like a fine choice. Unfortunately when we get to cleaning up we right now replace the entire queue and drop it, meaning we discard all blocks where as keeping the allocation for the first would be beneficial for workloads where only a small number of elements change. SeqQueue itself doesn't support that even if we did pop each element off one by one as it deallocates blocks the moment they become empty.

Might be worth exploring.