A workaround to prevent selector imbalance

This PR contains a configurable workaround to deal with selector imbalance. It can happen that some selectors get more work than other selectors and this leads to performance problems. 

With this work around the chance of this happening is reduced (still possible btw) and we need to have this in place for doing benchmarks for back pressure.

By default the work around is disabled and should only be used for benchmarking at the time being. In the future we need to find a real solution for this problem instead of this work around.
why not to make it a proper GroupProperty?