CBQE-0: improve logs in xdcr _online_upgrade

Change-Id: I53b9fef86a0d52eec2cb91859db5a18a682d6f04
Reviewed-on: http://review.couchbase.org/34725
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>