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/userdb/userdb.proto

@@ -2,6 +2,7 @@
 syntax = "proto3";
 
 package userdb;
+option go_package = "blitiri.com.ar/go/chasquid/internal/userdb";
 
 message ProtoDB {
 	map<string, Password> users = 1;