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 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

This is a breaking change insofar as that traewelling support is no longer
provided automatically, but must be enabled by providing a traewelling.de
application ID and secret in travelynx.conf. However, as traewelling.de
password login is deprecated and wil soon be disabled, travelynx would break
either way. So we might or might not see travelynx 2.0.0 in the next days.

Automatic token refresh is still todo, but that was the case for password
login as well.

Closes #64

travelynx now always calls checkout with the database-provided arr_eva,
so the edge case in which it provided a station that is not actually on the
route of a train can no longer appear.

This is consistent with other promises within travelynx