To improve delete performance, we want to process deletes asynchronously
once the message has been received from the satellite. This change makes
it so that storagenodes will send the delete request to a piece Deleter,
which will process a "best-effort" delete asynchronously and return a
success message to the satellite.
There is a configurable number of max delete workers and a max delete
queue size.
Change-Id: I016b68031f9065a9b09224f161b6783e18cf21e5
a785d37157