Improve extensibility of ScheduledThreadPoolExecutor and AbstractExecutorService

Users want the flexibility to both:
- create their own thread pool implementation, while reusing
  ThreadPoolExecutor or AbstractExecutorService
- create their own Future implementation, possibly reusing
  FutureTask

Practice has shown that the current implementations do not provide
enough hooks to allow users to do this.

###@###.### 2005-05-28 00:20:10 GMT
###@###.### 2005-06-11 23:29:12 GMT