Existing:
```go
type xlMetaV2 struct {
Versions []xlMetaV2Version `json:"Versions" msg:"Versions"`
}
```
Serialized as regular MessagePack.
```go
//msgp:tuple xlMetaV2VersionHeader
type xlMetaV2VersionHeader struct {
VersionID [16]byte
ModTime int64
Type VersionType
Flags xlFlags
}
```
Serialize as streaming MessagePack, format:
```
int(headerVersion)
int(xlmetaVersion)
int(nVersions)
for each version {
binary blob, xlMetaV2VersionHeader, serialized
binary blob, xlMetaV2Version, serialized.
}
```
xlMetaV2VersionHeader is <= 30 bytes serialized. Deserialized struct
can easily be reused and does not contain pointers, so efficient as a
slice (single allocation)
This allows quickly parsing everything as slices of bytes (no copy).
Versions are always *saved* sorted by modTime, newest *first*.
No more need to sort on load.
* Allows checking if a version exists.
* Allows reading single version without unmarshal all.
* Allows reading latest version of type without unmarshal all.
* Allows reading latest version without unmarshal of all.
* Allows checking if the latest is deleteMarker by reading first entry.
* Allows adding/updating/deleting a version with only header deserialization.
* Reduces allocations on conversion to FileInfo(s).
