added more diagnostics to xdcr xmem flushing

So that we can see exactly what was actually sent to remote end

Change-Id: I7bfaec8843c0071d31e1c7db372038605aabc5d6
Reviewed-on: http://review.couchbase.org/35715
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>