* X-User-ID header is processed to get user ID.
* Time of last request for a user is saved into DB.
* Time of last upload and download is stored for a user.
* Added DB migration to add columns into users table to store the times and app version.
* Backward fix of datatype of the deadline column in features table.
* Switched from crawshaw.io/sqlite to zombiezen.com/go/sqlite.
* Refactored DB handling.
* Used migration routine from zombiezen in favour of manual one.
* Runtime DB reinit simply deletes the db file and initializes the db anew.
Fix#6
* DB migration adds app_versions table for managing known app versions.
* /ping reads app version from User-Agent header and responds with latest version if the app is OKO and its version is older than the latest one stored.
* Added endpoint /app-versions which
* lists all known versions via GET
* adds/updates a version via POST
#3Fix#4
