avoid formatting upr packets if debug logging is disabled

Change-Id: I50a57fdebe249bf9a8e90d0223b43fb2fcf4a373
Reviewed-on: http://review.couchbase.org/38893
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>