MDMFSlotReadProxy: remove the queue

MDMFSlotReadProxy: remove the queue

This is a neat trick to reduce Foolscap overhead, but the need for an
explicit flush() complicates the Retrieve path and makes it prone to
lost-progress bugs.

Also change test_mutable.FakeStorageServer to tolerate multiple reads of the
same share in a row, a limitation exposed by turning off the queue.