glusterd,glusterfsd,libgfapi: Client op-version

This patch introduces op-version support for glusterfs clients.
Now, a client sends its supported op-versions during the volfile fetch request
and glusterd will return the volfile only if the client can support the current
op-version of the cluster.

Change-Id: Iab1f1f1706802962bcf27058657c44e8a344d2f6
BUG: 907311
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/4247
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>