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.

Closes #133

Decreases severity of #99
Fixes #108

Closes #126

Note that this is more of a hotfix -- the underlying bug is in
Travel::Status::DE::HAFAS