Allow calling client.persist on non-lazy data

Hi,
Not really sure if to label this a feature request, bug or more of a discussion. In our application of distributed it can sometimes occur that we try to call `client.persist` on data which isn't lazy, e.g. just regular numpy arrays. Currently this would throw an empty `AssertionError`. On our end the simple solution to this is to make sure that the data is lazy before trying to persist, or add some other logic to skip it entirely. However, with the increasing interoperability between dask and numpy maybe this would be a good enhancement to the client?