GTFS-rt stuff (mostly)

+ Uses the graph's timezone when parsing `StopTimeUpdates` to avoid being off by a few hours...
+ Updates PollingStopTimeUpdater to keep track of seen updates using a (TripId, timestamp) key, to avoid applying the same update multiple times.
+ Keeps track of the last seen feed's timestamp in `GtfsRealtimeHttpTripUpdateSource`.
+ Modifies GTFS-rt HTTP requests to also send an `If-Modified-Since` header, which contains the date of the last parsed feed, to avoid downloading the same feed multiple times.
+ Adds getters for `Graph#transitService{Starts,Ends}` and `TableTripPattern#scheduledTimetable`.