switch to xdcr-over-upr by default

Using XDCR_USE_OLD_PATH variable is the way to temporarily revert to
old (couchdb-base) path.

Change-Id: I049bf3aec46541ef11deb830c6b91c5fa7e0822d
Reviewed-on: http://review.couchbase.org/35533
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>