title: Dask prints warning to use client.scatter althought I'm using the suggested approach
body: <p>In dask distributed I get the following warning, which I would not expect:</p>
  
  <pre><code>/home/miniconda3/lib/python3.6/site-packages/distributed/worker.py:739: UserWarning: Large object of size 1.95 MB detected in task graph: 
    (['int-58e78e1b34eb49a68c65b54815d1b158', 'int-5cd ... 161071d7ae7'],)
  Consider scattering large objects ahead of time
  with client.scatter to reduce scheduler burden and 
  keep data on workers
  
      future = client.submit(func, big_data)    # bad
  
      big_future = client.scatter(big_data)     # good
      future = client.submit(func, big_future)  # good
    % (format_bytes(len(b)), s))
  </code></pre>
  
  <p>The reason I'm suprised is, that I'm doing exactly what the warning is suggesting:</p>
  
  <pre><code>import dask.dataframe as dd
  import pandas
  from dask.distributed import Client, LocalCluster
  
  c = Client(LocalCluster())
  dask_df = dd.from_pandas(pandas.DataFrame.from_dict({'A':[1,2,3,4,5]*1000}), npartitions=10)
  filter_list = c.scatter(list(range(2,100000,2)))
  mask = c.submit(dask_df['A'].isin, filter_list)
  dask_df[mask.result()].compute()
  </code></pre>
  
  <p>So my question is: Am I doing something wrong or is this a bug?</p>
  
  <pre><code>pandas='0.22.0'
  dask='0.17.0'
  </code></pre>