IOCP support (again)

[PyXiion]

Well, there's a lot of work to do to do that.

Right now I need to write/rewrite these things:

• io_notifier_iocp
• pipe_t: rename it to wake_source or something like that, and implement it on Windows via PostQueuedCompletionStatus and custom completion keys
• poll_info & poll_op
• timer_handle:
  1. std::multimap/std::set with timers, and calculate dynamic timeout for GQCS(iocp, ..., timeout_ms).
     On new timer wake IOCP queue with PQCS(..., TIMER_WAKE_KEY, nullptr).
     • Pros: one thread, no thread pool
     • Cons: heap usage, possibly bad performance with many timers, maybe precision loss
  2. CreateThreadpoolTimer + PostQueuedCompletionStatus from the callback
     • Pros: OS will manage it itself, high precision, simpler code
     • Cons: overhead for short timers, thread pool used
  3. CreateWaitableTimer + dedicated waiter thread
     • Pros: super duper simple
     • Cons: extra thread and overall bad