locks: Added an xdata-based 'cmd' for inodelk count in a given domain

Following is the semantics of the 'cmd':
1) If @domain is NULL - returns no. of locks blocked/granted in all domains
2) If @domain is non-NULL- returns no. of locks blocked/granted in that
domain
3) If @domain is non-existent - returns '0'; This is important since
locks xlator creates a domain in a lazy manner.

where @domain - a string representing the domain.

Change-Id: I5e609772343acc157ca650300618c1161efbe72d
BUG: 951195
Original-author: Krishnan Parthasarathi <kparthas@redhat.com>
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4889
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>