Provides a uniform Rails cache key for all `WayCost`s using their `id`.
Since the `id` field is option, raise an error if the `WayCost` doesn't
have one.

Refs #6097

If a `Route#cost` is missing the method shouldn't fail. Instead, it
should just pass the `WayCost` along without distance & time cost
values. This way we end up with a list of all `WayCost`s for all stop
areas, even the new ones that haven't been run through the TomTom
calculator yet.

Refs #6097

Gives us a way to take `Route#costs` and `WayCost`s created by
`StopAreasToWayCostsConverter` and merge the data together to obtain
complete `WayCost`s including distance and time values.

This will allow us to more easily interact with the Rails cache for
caching `WayCost`s.

Refs #6097

Use an `after_commit` instead of an `after_save`. The `after_save` could
cause intermittent problems due to Sidekiq starting before the
record/transaction actually gets committed to the database. In those
cases, the record wouldn't be found and cause an error.

With an `after_commit` callback, the record is sure to be committed
before Sidekiq takes over.

Additionally, limit the callback to `:create` and `:update` because
otherwise it will be active on `:destroy` by default also. This work
isn't relevant on destroy, so don't do it then.

Update the tests:

* Update labels
* Use `build` instead of `create` because we're saving the record at the
  end anyway so we don't need an object that starts out persisted
* Use `truncation: true` for these tests because otherwise the `commit`
  callback doesn't get called due to transactional teardown

Refs #6407