features/barrier: add barrier translator

gluster feature page:
http://www.gluster.org/community/documentation/index.php/Features/Server-side_Barrier_feature

Change-Id: Ia9f8802a54d1ffbd1cf789b80f5d30819bf65f64
BUG: 1060002
Signed-off-by: Varun Shastry <vshastry@redhat.com>
Reviewed-on: http://review.gluster.org/6928
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>