Gtfs realtime base

This pull request contains a base for further GTFS-rt development. It contains:
* Atomic processing of update messages
* An API parameter to ignore realtime updates
* A substantial part of Zsombors' work (adding trips and wheelchair accessibility excluded):
    - new gtfs-realtime.proto file
    - take different days into account when receiving and applying realtime updates
    - a base class for GTFS-rt update streamers such that new protocols can easily be added
    - some refactoring and bugfixes