*: proto refactoring

1. Add pbutil.MustMarshal and pbutil.MustUnmarshal 

2. Rename etcdserverpb.Info to etcdserverpb.WALMeta
The WAL package has the functionality to keep a piece of meta data (as byte array) and automatically store it into every wal file the WAL creates. The meta data is not part of the actual data committed into WAL. WAL does not need to understand the content of the data and how to decode/encode it.

The reason for saving the mate data is that etcd wants to give each WAL an identification (NodeID). When etcd recovers from WAL, it can also check/recover the identification. 

Therefor, in etcd's point of view the data is `the mate data of WAL` and it understands the content/structure of it. We define the meta data as a protobuf type, so we can do encoding/decoding. 

I think it is reasonable to have a protobuf type called `WALMetadata`. And it is only responsible for holding a structured meta data that etcd needs.