Make the I/O boss and worker Executors configurable (to accommodate SEDA threading models)

Heya Jeanfrancois

I'd like to be able to configure Nettosphere for our SEDA threading model, in which messages get delivered by the I/O threads into stages. In such models (the boss, I'd argue even in any other models as well) and (especially) the worker pools are beneficially small in size and should not by default span 2*(#of cores) threads.

Cheers, Christian.