Use -[GRPCCall setResponseDispatchQueue]

[wilhuff]

... to dispatch GRPC callbacks directly onto the Firestore worker queue.
This saves a double-dispatch and simplifies our logic.

[googlebot]

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[wilhuff]

PTAL. I've merged your changes from #812 and updated comments. I started into removing writeValue: and writesFinishedWIthError: from FSTStream but ultimately decided against it because those methods know too much about how the stream behaves to make them really good candidates for separation. As it stands now FSTCallbackFilter is just a filter and that's okay.

[mikelehen]

Thanks, LGTM!