addOperation(operation) {
        this.operations.push(async () => {
            try {
                this.actives++;
                await operation();
                this.actives--;
                this.completed++;
                this.parallelExecute();
            }
            catch (error) {
                this.emitter.emit("error", error);
            }
        });
    }
Add a operation into queue.

@param operation -