fix broken significant_change mechanism in ns_doctor

Change-Id: I1dea34281f603905ab29e7d8068cb770cef02e27
Reviewed-on: http://review.couchbase.org/38216
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>