CBQE-1521: infinite loop in XDCRBaseTest._wait_for_replication_to_catchup

Change-Id: Idbfe7b028e93dbc5303a0cc68dae514d43620fb7
Reviewed-on: http://review.couchbase.org/28355
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>