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/plugin/enumstringer/enumstringer.go

@@ -1,4 +1,6 @@
-// Copyright (c) 2013, Vastech SA (PTY) LTD. All rights reserved.
+// Protocol Buffers for Go with Gadgets
+//
+// Copyright (c) 2013, The GoGo Authors. All rights reserved.
 // http://github.com/gogo/protobuf
 //
 // Redistribution and use in source and binary forms, with or without
@@ -74,7 +76,7 @@ func (p *enumstringer) Generate(file *generator.FileDescriptor) {
 			continue
 		}
 		if gogoproto.IsGoEnumStringer(file.FileDescriptorProto, enum.EnumDescriptorProto) {
-			panic("old enum string method needs to be disabled, please use gogoproto.old_enum_stringer or gogoproto.old_enum_string_all and set it to false")
+			panic("Go enum stringer conflicts with new enumstringer plugin: please use gogoproto.goproto_enum_stringer or gogoproto.goproto_enum_string_all and set it to false")
 		}
 		p.atleastOne = true
 		ccTypeName := generator.CamelCaseSlice(enum.TypeName())