private-api: inconsistent measurement counts are returned

[hellais]

The values returned by these two API endpoints are inconsistent:

• https://api.ooni.io/api/_/country_overview?probe_cc=AZ
• https://api.ooni.io/api/_/countries

For some reason in one of the queries we are limiting the search to measurements newer than 2016-01-01, but for the other we are not: https://github.com/ooni/api/blob/56fe21b80a68a3024a3002f048f8922c4009b6b1/newapi/ooniapi/private.py#L1056 & https://github.com/ooni/api/blob/56fe21b80a68a3024a3002f048f8922c4009b6b1/newapi/ooniapi/private.py#L165

However, it's unclear to me why the one where the filter is applied leads to a higher measurement count, than the one without the filter.

[hellais]

This has been fixed