GTFS Realtime Stop Time Updates Timestamp type

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The [message StopTimeEvent](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-stoptimeevent) has a time field that is int64. Is there any use for this? If not, can we transition into a uint64 spec for the time field? This would be more uniform across all other timestamp fields in the GTFS Realtime Spec.

**Other timestamp fields in GTFS-RT, all as uint64**
* [message Modification.last_modified_time](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-modification)
* [message FeedHeader.timestamp](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-feedheader)
* [message TripUpdate.timestamp](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-tripupdate)
* [message VehiclePosition.timestamp](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-vehicleposition)
* [message TimeRange.start, TimeRange.end](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-timerange)