CBQE-0: RebalanceTask doesn't handle when rebalance hang

Change-Id: I565429c06056822f013e4b1832ad3454ca3ae1c1
Reviewed-on: http://review.couchbase.org/32850
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>