Return null on a non-200 http response
StoptimeUpdate: fix logging the number of applied updates
Keep track of update seen in the gtfs-rt feed
Add simple support for stoptime updates with delays
gtfs-rt: avoid null value if feed failed to fetch
Don't run StoptimeUpdater in an infinite loop

the PeriodicGraphUpdate should take care of running it as needed
Add simple tests for StopTimeUpdater (including Fix StopTimeUpdaterTest)
Remove parts of Zsombors' StoptimeUpdaterTest that are not implemented yet.