private void abortUnsubscribedSources(Throwable cause) {
            final int index = this.index;
            for (int i = index; i < sources.size(); i++) {
                sources.get(i).abort(cause);
            }
        }
Relay items in order from subsequent {@link StreamMessage}s as a single {@link StreamMessage} source.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatArrayStreamMessage.java