* It will first check for and return an idle worker.
* If all workers are busy, then it will try to create a new one up to the `max` worker count.
* If the max worker count is reached, the emitter will emit a `FullPool` event and it will fall back to using a round robin algorithm to distribute the load.
+ *
+ * @returns Cluster worker.
*/
protected chooseWorker (): Worker {
let worker: Worker | undefined