Change names to StreamedUnary, ServerUnaryStreamer, etc. Use a templated...

Change names to StreamedUnary, ServerUnaryStreamer, etc. Use a templated method handler since most code shared between the new StreamedUnary and the existing BidiStreaming. Eliminate the separate enum case for streamed unary. Return a status failure if a StreamedUnary method handler doesn't actually do a write (since that is
violating the appearance of unary-ness)