Time-keyed routes resurrected

This puts back into place the idea of having a when-to-list of routes
mapping on each of the tracks. The motivation with this is twofold:

- the 'when' value *can* be unique to the particular station
- later, if we have a lot of 'when'='suspended' or 'when'='situational'
  routes, then callers shouldn't have to sift through them all if those
  are not interesting