Major refactoring for event loops.

 - Status listeners no longer receive a Transport as this is usually
   useless reference count traffic.
 - The OnHangup and OnError callbacks have been merged.
 - Pollers now inherit from IODispatcher, since event loops act as
   pollers.
 - The polling impls have been refactored around a new concept of
   "proxy" listeners, which are meta-listeners responsible for
   forward events on to other listeners.
 - New classes for event queues and event loops have been added. These
   allow creating custom message loops and priority queues for polled
   events.
Lots of file renames for brevity.