glusterd: make volinfo a refcnt'ed object.

Add glusterd_volinfo_remove(..) which removes @volinfo from the list
of volumes in the cluster and performs an unref on @volinfo

Change-Id: I5f546ca58f61bc334ab1bab4c51c4a21e1f66161
BUG: 1038051
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/6521
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>