title: Thread-Pool with multiple limits
body: <p>I want a Thread-Pool that provides a maximum of X Threads to process tasks, so far no problem. However each submitted Task can specify an IO-Target which is specifically limited (say Y).</p>
  
  <p>So a submitted IOTask returns the target "google.com" with limit 4 (Y) and the pool has a global limit 16 (X). I want to submit 10 google.com-tasks where only 4 are processed in parallel and the pool has 12 threads free for other tasks.</p>
  
  <p>How can I achieve this?</p>