glusterd/snapshot : Barrier code integration with snapshot codebase.

As we have new barrier translator in place, we are making use of
that during snapshot phase.

During snapshot create (pre-commit), we enable the barrier feature
and after the commit we disable it.

Change-Id: I94212b1c06b0d9b12255ee98313e2d8549b34b17
BUG: 1061685
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/7561
Reviewed-by: Kaushal M <kaushal@redhat.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>