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

vendor/github.com/gogo/protobuf/gogoreplace/main.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+)
+
+func main() {
+	args := os.Args
+	if len(args) != 4 {
+		fmt.Println("gogoreplace wants three arguments")
+		fmt.Println("	gogoreplace oldsubstring newsubstring filename")
+		os.Exit(1)
+	}
+	data, err := ioutil.ReadFile(args[3])
+	if err != nil {
+		panic(err)
+	}
+	data = bytes.Replace(data, []byte(args[1]), []byte(args[2]), -1)
+	if err := ioutil.WriteFile(args[3], data, 0666); err != nil {
+		panic(err)
+	}
+}