Add support for deleting items from the storage backends
Description
IN preparation for CAS cleanup, storages will need to be able to delete entries from themselves. This MR adds a delete_blob
method to the StorageABC
and implementations for each storage backend. There is also a test for deletes.
NOTE: This isn't implemented for the "remote storage" backend because there isn't a deletion method in the CAS protocol specification.
Edited by Rohit Kothur