cluster/dht: introduce dht locking api.

Change-Id: I41389ba91951d3e63e617aa32cd0bee848261c72
BUG: 1140348
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/8521
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
(cherry picked from commit a1fe3d72e373bf0deaed152842d12d94bb9129dc)
Signed-off-by: Nithya Balachandran <nbalacha@redhat.com>
Reviewed-on: http://review.gluster.org/8722
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>