do not log upr window update packets in upr_proxy

Because they're not interesting for purpose of analyzing high-level
replication flow and because they're quite frequent.

Change-Id: Iad5ef8d5bbff8c75e9c75ff7b3c569e7d2b9b7a8
Reviewed-on: http://review.couchbase.org/38998
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>