Spawn new worker threads (not above self.max).

def grow(self, amount):
        """Spawn new worker threads (not above self.max)."""
        for i in range(amount):
            if self.max > 0 and len(self._threads) >= self.max:
                break
            worker = WorkerThread(self.server)
            worker.setName("CP Server " + worker.getName())
            self._threads.append(worker)
            worker.start()