Add `start_time` to uniquely identify journeys.

[lenaschimmel]

We currently use trip_id and start_date to identify specific trips. As @derhuerst pointed out here and in a private conversation, this is not enough to identify journeys in frequency-based trips. This is also clarified in the GTFS-RT spec.

We need to:

• [x] add start_time column to our db_schema here
• [x] update the update_statement and insert_statement in per_schedule_importer
• [x] Re-import all data so that start_time is guaranteed to contain data
• [x] Use the start_time to match rows in curves.rs

This issue has low priority because (to our current knowledge) no service in Germany has frequency-based trips.

Anyway, we need to check if this issue also affects "Verstärkerfahren", that is, additional trips that are not found in the schule but resemble scheduled trips and might re-use their trip_id.

[lenaschimmel]

This is now fixed in the code and just need some testing.