GlusterFS: Lock on a per-volume basis

This allows operations that do not conflict with each
other (i.e. are on different volumes) to run concurrently.
The prior locking scheme was too coarse and essentially
made the driver single-threaded.

Change-Id: Ib048c21f66926cb96ae02ced4bd16a752d52b301
Merge "GlusterFS: Lock on a per-volume basis"