[hack] Add autocomplete to serverless IDE
[hack] Add autocomplete to serverless IDE

Summary:
Now that the symbol index is fully hidden behind a single "initialization" construct, let's move it into serverless IDE.

This means that the serverless IDE should call `SymbolIndex.initialize` and be able to hand requests off to the autocomplete pathway.

Items to note with this diff:
* This diff only handles completion requests; it does not handle resolve requests.
* If you have HH server disabled, and nuclide sends a resolve request, the client will lock up since it will never get a response.
* If HH server is up and running, autocomplete requests will be sent to HH server instead of serverless IDE.

Reviewed By: pittsw

Differential Revision: D16036603

fbshipit-source-id: e0b956e1cbf56e925b76b9c01a846bed4f2d08ef

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