glusterd: Add a barrier brick-op

This patch introduces a new 'barrier' brick-op which will be used to
activate/deactivate the barriering on the bricks. This includes
barriering in the barrier xlator and in the changelog xlator. All the
required code has been including a bricks select function, a payload
builder and a brick-op handler.

Change-Id: I91d9d77f691c2e89823f7dc4e84900ec40dc4dd2
BUG: 1060002
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/6943
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>