Threading in the new API.

[Alan Conway <ac...@redhat.com>]

Here's a propsoal for threading in the new API. It is fully decoupled from the 
rest of the API and supports  thread pools and select/poll integration.

/** A group of sessions that can be dispatched by a common set of threads. */
class SessionDispatcher {
     void add(Session);
     void remove(Session);

     class InterruptedException ...;

     /**
      * Dispatch messages to message listeners on all session in the group.
      * Thread safe: dispatch can be called concurrently by multiple threads,
      * message listeners on the same session will be dispatched sequentially.
      *
      *@param timeout Dispatch for up to the timeout.
      *@throws InterruptedException if dispatching is interrupted.
      **/
      bool dispatch(Duration timeout);

     /**
      * Throw InterruptedException to all threads in dispatch()
      */
      bool interrupt();

     // Unix only, windows would return a WaitableObject or somesuch.
     /**
      * Returned fd will become readable when there is work to be dispatched.
      */
      int getFd();
};

Now you can trivialy create a thread pool to service a group of sessions:
  - create a SessionsDispatcher, add your sessions to it.
  - start N threads calling SessionsDispatcher::dispatch(INFINITE).
  - when you want to stop, call SessionsDispatcher::interrupt()  and join your 
threads.

For select/poll integration:
  - add SessionsDispatcher::getFd() to your select set.
  - When it becomes readable do SessionDispatcher::dispatch(0)

Thoughts?

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