probe: add config option to automatically delete measurements older than X days

[hellais]

It has been reported in https://github.com/ooni/probe/issues/1950 & https://github.com/ooni/probe/issues/1927 that it's a problem for users that run OONI Probe unattended for a long time that the database size just keeps growing.

Currently whenever a measurement is uploaded we delete the raw JSON on disk and store in the local database the metadata about the measurement so that we can display it in the result listing. If the measurement is not uploaded, in addition to the metadata in the DB, we also retain the raw JSON.

What is being proposed here is that, in addition to deleting the raw JSON once it’s uploaded (which is already done), we also delete the metadata from the database after some amount of time. If we do this, those older measurements will effectively disappear from the users app and will not be retrievable in any other way anymore.

We should have an option in the settings that will automatically delete measurements from the DB that are older than some predefined date interval.

This is also related to: https://github.com/ooni/probe/issues/1031