feature/barrier: Add statedump support

This patch adds statedump support for barrier. This currently dumps
barrier xlators private information and the queue of barriered fops.

Change-Id: I273eb6e676db02c40c363feeff58a79737dc041e
BUG: 1060002
Reviewed-on: http://review.gluster.org/7136
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>