cluster/distribute: Prevent spurious multiple defrag crawls

In dht_notify, we used to create a thread to start defrag
crawls after we had heard from all child subvols.
This was in-correct, as a later event, could also trigger the
crawl again(due to the fact that all subvols had responded).

The fix is to make sure, the thread is started only once after
all subvols have responded the first time

Change-Id: Ifc2978b9dc866af2395b79911eca50ab38ff9457
BUG: 916449
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4587
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>