Replace protobuf output formats with gRPC.

[gbrail]

gRPC may make it easier for transicator clients to consume content, especially since we already did some non-GRPC work to make protobufs work. If we use gRPC it will work better for the clients. It also may help changeserver scale.

The JSON-based API will never go away, however.

Plus gRPC is the thing to do at Google and will enable other stuff that we need in the future.