CBQE-1053: optimizations in biXDCR.py/uniXDCR.py

Change-Id: I22dd6626cfb46f39b1bf89eb0f2147ea25986953
Reviewed-on: http://review.couchbase.org/24776
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>