arrivals_departures/connections: move id column into separate view

[derhuerst]

Currently, queries on arrivals_departures & connections are slow as soon as there are frequencies-based rows in there. This is because the frequencies_it – which is needed for arrival_departure_id/connection_id – calculation uses a row_number() over a PARTITION BY.

This PR

• moves these expensive columns into separate views arrivals_departures_with_ids/connections_with_ids;
• adapts arrival_departure_by_arrival_departure_id/connection_by_connection_id to use the new views;
• adds benchmarking for the case described above.