Retain logger configuration

by saving a strong reference to them.

I was encountering strange behaviour with the loggers. They start off by respecting the logger configuration, but after some time revert back to default behaviour.

After spending a day chasing this, I realized that the Java platform doesn't keep strong references to Loggers (by design). So after a GC cycle, the loggers get destroyed and get re-created upon the next log call. The newly created loggers don't have any configuration applied to them.

This PR is just a quick hack. A long term solution might be to use a logging framework such as `slf4j` which probably will take care of this. I see that you have already begun some work there.