Use google.golang.org/protobuf instead of github.com/gogo/protobuf

This change replaces github.com/gogo/protobuf with
google.golang.org/protobuf, except for the code generators.

All proto-encoded structs are now generated from .proto files,
which include ttrpc.Request and ttrpc.Response.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

proto/status.proto

@@ -0,0 +1,5 @@
+syntax = "proto3";
+
+message Status {
+	int32 code = 1;
+}