Improve refcounted block reclamation

[LeeSmet]

Currently deletes are handled in the delete call. It would be better if the files to remove are just moved to a separate collection, so that a worker thread can take care of this in the background. This would decrease the time it takes for delete calls to just move the files in the meta db. This would also be a big improvement for bucket deletes, as we can simply rename the bucket and reclaim it later. Right now, creating a bucket with the same name as a recently deleted bucket will fail until the bucket is completely deleted.