async triggerOutgoingHandlers() {
        let buffer;
        do {
            if (this.executingOutgoingHandlers >= this.concurrency) {
                return;
            }
            buffer = this.outgoing.shift();
            if (buffer) {
                this.triggerOutgoingHandler(buffer);
            }
        } while (buffer);
    }
Try to trigger a outgoing handler for every buffer in outgoing. Stop when
concurrency reaches.