glusterd: Made volume-sync use synctask framework.

Change-Id: I048aac2af4d9da9ed541d3756fefefbb2a29198e
BUG: 852147
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4489
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>