Support for KMeans with an async Client

Hi,

I'd like to ask if It's possible to add a version of KMeans that works with an async client.

Here is an example that should work:
```python
import distributed
import dask_ml
import dask_ml.datasets
import dask_ml.cluster

client = distributed.Client(asynchronous=True)
await client

X, y = dask_ml.datasets.make_blobs(n_samples=10000000,
chunks=1000000,
random_state=0,
centers=3)
km = dask_ml.cluster.KMeans(n_clusters=3, init_max_iter=2, oversampling_factor=10)
km.fit(X)
```

Thank you