public void process(DocumentOperation documentOperation) {
        process(documentOperation, null);
    }
Asynchronously process the given document operation using the processing
chain of this service.

@throws RuntimeException caused by a QueueFullException if this DocprocService has a bounded input queue and the queue is full
@throws IllegalStateException if this DocprocService is not accepting new incoming processings
docproc/src/main/java/com/yahoo/docproc/impl/DocprocService.java