Make sure stats are always consistent when handling a task result

Hi, thank you for this great library.

I'm using it with the following syntax:
```
const pool = workerpool.pool(__dirname + '/import-worker-fast-v3.js', {
minWorkers: 'max',
maxWorkers: 8,
workerType: 'thread',
});
```
During execution of a loop, I check `pool.stats()` and I get the following output:
```
active: 7
pendingTasks: 9786
busyWorkers: 7
idleWorkers: 1
totalWorkers: 8
```

One worker is always idle. Am I doing something wrong?