Inconsistent treatment of non-string worker names

The treatment of numeric worker names is inconsistent:

- ``Client.retire_workers(names=[...])`` maps worker names to strings, so a worker may be called ``1`` and you can pass ``"1"`` to the method, or vice versa
- ``Client.submit(..., workers=[...])`` does not do the mapping; ``1`` and ``"1"`` are different
- ``Client.scatter(..., workers=[...])`` does not do the mapping either
- ``Client.replicate(..., workers=[...])`` silently fails when passing string names and crashes when passing numeric names
- ...other methods?

(note: scatter/replicate to specific workers are pending discussion and redesign within the scope of the Active Memory Manager)

xref:
- #2949

@mrocklin in #2949 you mentioned support for CLIs. Were you referring to any specific CLI? Is the feature still desirable?
CC @gjoseph92