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?