Until now, tripIDs were cached based on station and departure timestamp.
These are identical for any two trains departing at the same time at the same
station, leading to one of those getting being assigned a wrong tripID.

From now on, only the JSON reported by transport.rest is cached -- tripIDs
are always recomputed based on it.

nginx cache keys depend on session data, so misbehaving clients
(ignoring the refresh dely and continuously requesting a new departure board)
with renewed session data could not be cached, impacting DBF performance

Closes #6