MB-100: fix timeouts in rebalanceXDCR suite

Change-Id: I44587aa44c3eef5c8fb0eeacc6e06afe4a3675e9
Reviewed-on: http://review.couchbase.org/24851
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>