class input_async_mplex :Creates an asynchronous input channel reading from the multiplex controller.
?onshutdown:onshutdown_in_spec -> ?buffer_size:int -> multiplex_controller ->
buffer_size: The size of the internal buffer. By default unlimited.
Note that the engine is done when the input channel is closed.
The socket is not shut down, and the underlying file descriptor
is not closed! You can define the
shutdown callback to do something
in this case.