Remove Try rewrapping from ServerGeneratorStream
Remove Try rewrapping from ServerGeneratorStream

Summary:
- Create `StreamElementEncoder` interface in `StreamCallbacks.h` with three `()` operator overloads
- Define concrete class `StreamElementEncoderImpl` in `GeneratedCodeHelper.h`
- Pass pointer to static instance of `StreamElementEncoderImpl` instead of `encode_stream_element()` function pointer

Reviewed By: iahs

Differential Revision: D30235486

fbshipit-source-id: b1d4c2e1c7f5a875f0964729e28cc356af03efc9

ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp