CBQE-0: improve logs in xdcr _online_upgrade

Change-Id: I920c6cbf6fc0e75499632544cebd9dcd1f0d3aad
Reviewed-on: http://review.couchbase.org/34723
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>