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/install-protobuf.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+
+set -ex
+
+die() {
+    echo "$@" >&2
+    exit 1
+}
+
+case "$PROTOBUF_VERSION" in
+2*)
+    basename=protobuf-$PROTOBUF_VERSION
+    ;;
+3*)
+    basename=protobuf-cpp-$PROTOBUF_VERSION
+    ;;
+*)
+    die "unknown protobuf version: $PROTOBUF_VERSION"
+    ;;
+esac
+
+cd /home/travis
+
+wget https://github.com/google/protobuf/releases/download/v$PROTOBUF_VERSION/$basename.tar.gz
+tar xzf $basename.tar.gz
+
+cd protobuf-$PROTOBUF_VERSION
+
+./configure --prefix=/home/travis && make -j2 && make install