Add return value for "/trackservice" endpoint, improve logging

Public Controller

• The return value for the "public_controller.list_tracks" method is missing. It should return a list of Track. Compare implementation in internal_controller.list_playlog. The actual logic was there before, but got lost during a merge mistake. Check the recent git history to recover it.
• Improve logging: There should be no log when from_date and to_date are missing. When these parameters are available, but in invalid format, there should be a warn log entry and the actual values skipped.
• Add test case (requires #5 (closed) as a basis)

Internal Controller

• internal_controller.list_playlog Improve logging: There should be no log when from_date and to_date are missing. When these parameters are available, but in invalid format, there should be a warn log entry and the actual values skipped.
• Add test case (requires #5 (closed) as a basis)