*: proto refactoring

1. Add pbutil.MustMarshal and pbutil.MustUnmarshal 

2. Rename etcdserverpb.Info to etcdserverpb.WALMeta
@jonboulle i didn't say it clearly enough. if the cluster doesn't have any update or change, etcd could always calculate out the same id based on the command line. so the id from command line and the id from WAL should always be the same. i compare them to make a check.
it is good to delete it now to avoid further confusion.