cli,glusterd: Implement 'volume status tasks'

oVirt's Gluster Integration needs an inexpensive command that can be
executed every 10 seconds to monitor async tasks and their parameters,
for all volumes.

The solution involves adding a 'tasks' sub-command to 'volume status'
to fetch only the async task IDs, type and other relevant parameters.
Only the originator glusterd participates in this command as all the
information needed is available on all the nodes. This is to make the
command suitable for being executed every 10 seconds.

Change-Id: I1edc607baf29b001a5585079dec681d7c641b3d1
BUG: 1012346
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/6006
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>