glusterd: Improve rebalance handling during volume sync

Glusterd will now correctly copy existing rebalance information when a
volinfo is updated during volume sync. If the existing rebalance
information was stale, then any existing rebalance process will be
termimnated. A new rebalance process will be started only if there is no
existing rebalance process. The rebalance process will not be started if
the existing rebalance session had completed, failed or been stopped.

Change-Id: I68c5984267c188734da76770ba557662d4ea3ee0
BUG: 1036464
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/6334
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>