Disable strack trace lookup for JUL logging

Since the proper way to format JUL is to provide a custom formatter like we've done with VertxLoggerFormatter, I propose we just disable stacktrace lookup altogether (since it's slow) and use the logger name by default. 

If anyone wants location information in their logs, it would be fairly trivial to do so with a custom formatter. Or maybe even better, we provide another formatter in vertx that does it ?
I know it's not something you added, but just an observation that creating a new DateFormat for every record logged is pretty inefficient. Better would be to use a thread local date format.