function onflush() {
			worker.removeEventListener('message', onmessage, false);
			onend(outputSize, crc);
		}
inflate/deflate core functions
@param worker {Worker} web worker for the task.
@param initialMessage {Object} initial message to be sent to the worker. should contain
  sn(serial number for distinguishing multiple tasks sent to the worker), and codecClass.
  This function may add more properties before sending.