Parent issue: #401
Relates to: #401 (comment)

• Add a warning when the configuration does not enable any service (UDP tracker, HTTP tracker, or tracker API). It does not make sense to run the app without services enabled.
• Write the final used config to the logs when the tracker starts. We are only writing the config values when the source is an env var. We should do it always and print out the final configuration after emerging sources (defaults -> TOML file -> env vars).