Update protobuf library to v2

There is a new protobuf library (and corresponding code generator) for
Go: google.golang.org/protobuf.

It is fairly compatible with the previous v1 API
(github.com/golang/protobuf), but there are some changes.

This patch adjusts the code and generated files to the new API.

The on-wire/on-disk format remains unchanged so this should be
transparent to the users.

internal/protoio/testpb/dummy.go

@@ -1,3 +1,3 @@
 package testpb
 
-//go:generate protoc --go_out=. testpb.proto
+//go:generate protoc --go_out=. --go_opt=paths=source_relative testpb.proto