MB-8668: unbreak saving of xdcr checkpoints

Change-Id: I679cf74c1cb2e82ed893c9a22dbad82904ce97bc
Reviewed-on: http://review.couchbase.org/43223
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>