Instead to implement own logic how the database scheme should be updated
or migrated to a newer or older version flucky use now instead the
go-migrate package.
Add additional functions to the repository to add or update devices,
sensors or measured values. Furthermore the test has been adapt to the
new functions.
Add timezone for the columns creation_date and update_date, otherwise
it's difficult to compare the times correctly in the unit test.
Furthermore the default value of the creation_date will now be defined
by the postgres implementation of the database interface.
