Enable "Traverse" for client.persist(..)

Hello,

I am passing a python dict where the values are delayed objects.
Calling client.persist(my_dict) results in below error;

```
assert all(map(dask.is_dask_collection, collections))
```
source: https://github.com/dask/distributed/blob/master/distributed/client.py#L2882

I was wondering if it was possible to support traverse=True for client.persist(..) method, as it's supported for client.compute(..).

I imagine the "if traverse.." block from client.compute could also fit for client.traverse?
source: https://github.com/dask/distributed/blob/master/distributed/client.py#L2750-L2756

Thanks all!

Best,
Jenna