Preserve per-stoptime predictions from GTFS-realtime updates.

GTFS-realtime provides feed producers with the ability to supply multiple arrival/departure predictions per trip.  This can be used to enhance the quality of passenger information where it is known/predicted in advance (whether from manual input or a predictive algorithm) that a trip will further gain/lose time at future stops.

This PR adds support for preserving those per-stoptime predictions from GTFS-realtime sources and passing them through the TDS.  It also adds support for exposing those per-stoptime predictions in the GTFS-realtime exporters in the API webapp and TDS.

The API webapp and legacy Web interface do not make direct use of these per-stoptime predictions, but they can be used by the OBANYC SIRI API and Web UIs with a suitable `PredictionHelperService` (patch forthcoming).

This also provides the necessary infrastructure to preserve per-stoptime updates from SIRI VM sources, but that is not implemented here.

Finally, this PR augments the TDS GTFS-realtime export with a text-format Protocol Buffers debug mode, activated by calling the endpoint with `?debug=1`.