Output GTFS-RT statistics to log periodically

Prints out better statistics about GTFS-RT TripUpdate handling.

To be completed by pull request submitter:

[ ] issue: Link to or create an issue that describes the relevant feature or bug. Add GitHub keywords to this PR's description (e.g., closes #45).
[ ] roadmap: Check the roadmap for this feature or bug. If it is not already on the roadmap, PLC will discuss as part of the review process.
[ ] tests: Have you added relevant test coverage? Are all the tests passing on the continuous integration service (Travis CI)?
[X] formatting: Have you followed the suggested code style?

To be completed by @opentripplanner/plc:

[ ] reviews and approvals by 2 members, ideally from different organizations
[ ] before merging: add a bullet point to the changelog file with description and link to the linked issue
[ ] after merging: update the relevant card on the roadmap

HSLdevcom / OpenTripPlanner