wal: add marshaler interface

1. Add a Marshaler Interface into raft package
```go
type Marshaler interface {
 	Marshal() ([]byte, error)
 	MarshalType() int64
 }
```

2. Use Marshaler in wal pkg. simplify things a lot.

3. WARNING: remove head checking when writing node.Info.
I was suggesting the interface be moved to wal. I understand you can't add methods to types outside the wal package.