cluster/dht: Added code to capture races in dht-lookup path

Change-Id: I9270d2d40ebd4b113ff961583dfda7754741f151
BUG: 1138385
Signed-off-by: Venkatesh Somyajulu <vsomyaju@redhat.com>
Reviewed-on-master: http://review.gluster.org/8430
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/8668
Tested-by: Vijay Bellur <vbellur@redhat.com>