vendor: Update github.com/gogo/protobuf

Also tweaks the proto definitions:

 - [packed=false] on the block_indexes field to retain compat with
   v0.14.16 and earlier.

 - Uses the vendored protobuf package in include paths.

And, "build.go setup" will install the vendored protoc-gen-gogofast.
This should ensure that a proto rebuild isn't so dependent on whatever
version of the compiler and package the developer has installed...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3864

build.go

@@ -400,6 +400,8 @@ func setup() {
 		fmt.Println(pkg)
 		runPrint("go", "get", "-u", pkg)
 	}
+
+	runPrint("go", "install", "-v", "./vendor/github.com/gogo/protobuf/protoc-gen-gogofast")
 }
 
 func test(pkgs ...string) {