asyncio: SSL transports now clear their reference to the waiter

asyncio: SSL transports now clear their reference to the waiter

* Rephrase also the comment explaining why the waiter is not awaken immediatly.
* SSLProtocol.eof_received() doesn't instanciate ConnectionResetError exception
  directly, it will be done by Future.set_exception(). The exception is not
  used if the waiter was cancelled or if there is no waiter.