io-stats: Adding private data dumping method

For the glusterfsiostat tool to be able to gather stats about mounted volumes
from meta xlator, private information in the io-stats xlator needs to be dumped
in the .meta folder.

Added functionality for total data being read/written to be dumped along with
latency related information for all fop functions present in io-stats.

Change-Id: I75486f0ca361844a643861789f6c1406f439674d
BUG: 1130023
Signed-off-by: Vipul Nayyar <nayyar_vipul@yahoo.com>
Reviewed-on: http://review.gluster.org/8244
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/8696