CBQE-0: fix incremental_rebalance_in_out_with_mutation_and_expiration

seems test hangs http://qa.hq.northscale.net/job/centos-64-2.0-new-rebalance-mixed-cluster-P0/129/consoleFull

Change-Id: I494688f35309a21b976e497980673edb5e6faac5
Reviewed-on: http://review.couchbase.org/27103
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>