[ServiceWorker] Add verbose mode for debugging

[Neet-Nestor]

Using service worker sometimes is tricky, especially when you are trying to debug an issue. This PR adds additional debug level logs for printing the entire event trace for easier debugging.

Broswer console in default logging level:

Broswer console in verbose logging level:

[tqchen]

would be great to change to logLevel, we should also consider allow customized logger interface in future, but not something we have to do now.

We can do conditional execution inside the logger(checks logLevel)

logger.info("message", structured_data)

[Neet-Nestor]

@tqchen Updated. For now, I'm using the default console.debug() method which will only ouptut if broswer log level is set to verbose.