class input_async_mplex :?onshutdown:onshutdown_in_spec -> ?buffer_size:int -> multiplex_controller ->async_in_channel_engine
 onshutdown: See output_async_mplex.
 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.