checkShutdown() {
        if (!this.shutdownInProgress) return;

        let busyWorkersCount = 0;

        this.workersQueue.forEach(worker => {
            if (worker.isBusy()) {
                busyWorkersCount += 1;
            } else {
                worker.stop();
            }
        });

        if (busyWorkersCount === 0) {
            this.isTerminated = true;
            this.emit('terminated');
        }
    }
Kills the workers when they're all done.

@private