Invoked before the last copy of the pipeline is about to be discarded, to give {@link Tube}s a chance to clean up any resources.
This can be used to invoke {@link PreDestroy} lifecycle methodson user handler. The invocation of it is optional on the client side, but mandatory on the server side.
When multiple copies of pipelines are created, this method is called only on one of them.
@throws WebServiceException If the clean up fails, {@link WebServiceException} can be thrown.This exception will be propagated to users (if this is client), or recorded (if this is server.)