Broken by 993f6be6

Squashed commit of the following:

commit d60c7d3c98d88a8f5b0e3ced6c11b56053e1e44b
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Jan 27 20:22:46 2020 +0100

    fix bugs related to users without past journeys

commit 707fcc937ac7f6bc3dc29024273f5e74963f7f15
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Jan 27 20:19:14 2020 +0100

    work around Cache::file turning floats into strings

commit 55831121eb30bc30ed20134bbb48e4bee9772feb
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Jan 27 19:43:29 2020 +0100

    store journey polylines for later use

commit 1971d511037ff2b8fbc9699cb98e4f8fd51261e5
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Sat Jan 25 16:49:48 2020 +0100

    set preliminary database schema for polyline storage.

    deduplication will follow at a later stage

When a user changes their destination station or checks into a new train
while work is updating data for their in_transit entry, values for no longer
valid database entrie would be entered.

As a side effect, this turns the response of GET /journey/edit (which should
never happen in the first place) from an exception into a 404 error (#36)

The issue manifested itself by the ioloop using 100%cpu without making any
progress.