Use flatbuffers in grpc instead of protobuf fields

[rydrman]

This will still allow the server to accept old-style messages but always responds with flatbuffers. I'm mainly just keeping the other code around so that I can make a build internally to handle the transition - I thought about adding additional fields/etc to support a transition more easily but it doesn't seem worth the effort given that only we are using the server so far. I'm open to ideas if it seems like there's an desire and easy way to do this, but otherwise I'll circle back later and remove the rest of the old format.

[jrray]

It's still true we're not using the server stuff, I'll go with your gut on this.