| ...header_n     | msgp bin array              | Header of last version
| ...metadata_n   | msgp bin array              | Metadata of last version

Each header contains a mspg array (tuple) encoded object:

xlHeaderVersion version == 1:

```
//msgp:tuple xlMetaV2VersionHeader
type xlMetaV2VersionHeader struct {
 VersionID [16]byte  // Version UUID, raw.
 ModTime   int64     // Unix nanoseconds.