Add basic support for ADDED trips

The Graph is updated in memory, with patterns valid for a single day.
The new edges are not removed from the graph for now.

- add getAllStops(), getAllRoutes() to TransitIndexService
- modify Updates to store an AgencyAndId for stops
- add ServiceIdToNumberService#addService to index new services
- add TableTripPattern#traversable, which is used to disable traversing
  TransitBoardAlight edges while the graph is being updates
- modify GTFSPatternHopFactory to support creating patterns on an
  existing graph
Support removing ADDED trips from the graph

TripPatterns which were active on the day-before-yesterday are removed
automatically.
Use ServiceDates for determining timetable validity when resolving

- ServiceDays also store a ServiceDate so it doesn't have to be
  recreated.
- Purge expired data daily, based on the serviceDate.
Update GTFS-rt protobuf definitions

+ add wheelchairAccessible/deviated extension