Stoptimes should return tripId #3589

Expected behavior

/index/stops/.../stoptimes call is documented to return, among other fields, a tripId. It would be very useful to know when a coming bus would reach other stops in its itinerary.

Observed behavior

As I see in TripTimeMapper.java, the tripId field is the only one that's not set.

Version of OTP used (exact commit hash or JAR name)

otp-2.0.0-shaded.jar

Steps to reproduce the problem

Start OTP and query something like localhost:8080/otp/routers/default/index/stops/1:828/stoptimes.