cluster/dht: Ignore decommissioned subvol in overlap optimization

Change-Id: Ib727948c6e21b19fd509f258ff0aea1c5d1a84d1
BUG: 966845
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/5056
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-on: http://review.gluster.org/6517
Reviewed-by: Shishir Gowda <gowda.shishir@gmail.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>