Complete transition to structured logging for aggregator

[bernhardreiter]

Using structured logging as introduced with #530 is an advantage but removing --verbose may be considered an API change and thus would trigger a major release (e.g. 4), maybe changing the logging output is also a change in behavior.

This issues is to consider the implications and plan when to remove verbose and how to release the addition. A variant could be to let verbose set a logging level and do a minor release if the log output is not considered part of significant interface.