features/changelog: Barrier in changelog during snapshot.

        Changelog barriers unlink, rename, rmdir fops on barrier 'on'
        notification from glusterfsd mgmt layer and unbarriers the
        same on barrier 'off' notification during snapshot.

        Please see the following link for more details.
        http://www.gluster.org/community/documentation/index.php/Changelog_Design_changes_for_snapshot

Signed-off-by: Kotresh H R <khiremat@redhat.com>

Change-Id: Iea9c62fafc86242f9404e03679b1941aa9c88c9a
Signed-off-by: Kotresh H R <khiremat@redhat.com>
Reviewed-on: http://review.gluster.org/7415
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Varun Shastry <vshastry@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>