this avoids a race condition when the worker requests all current checkins and
takes a while before getting to update the data

This is in preparation for supporting multiple HAFAS backends, and possibly
EFA and RIS::Journeys.

This is in preparation for HAFAS wagonorder support

this avoid race conditions when a user changes their checkin between the time
of request (old trip id) and the time of data update (new trip id). These
could lead to funny effects such as buses suddenly having S-Bahn carriage
formations.

Decreases severity of #99
Fixes #108