Allow client wait_for_workers to wait for an absolute number of workers

Currently `Client.wait_for_workers` waits until the cluster has at least the number specified. So if you have three or more workers and call `client.wait_for_workers(3)` it will continue.

There are scenarios where you may want to wait for an absolute number of workers, for example when scaling down. It would be great to allow `wait_for_workers` to handle this too.

One solution could be to introduce an `absolute` keyword that defaults to `False`.

```python
client.wait_for_workers(7) # Waits for at least 7 workers

client.wait_for_workers(7, absolute=True) # Waits for exactly 7 workers
```