Improve performance on multiple versions (#13573)

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).

cmd/metacache-entries.go

@@ -148,11 +148,15 @@ func (e *metaCacheEntry) isLatestDeletemarker() bool {
 	if !isXL2V1Format(e.metadata) {
 		return false
 	}
+	if meta, _ := isIndexedMetaV2(e.metadata); meta != nil {
+		return meta.IsLatestDeleteMarker()
+	}
+	// Fall back...
 	var xlMeta xlMetaV2
-	if err := xlMeta.Load(e.metadata); err != nil || len(xlMeta.Versions) == 0 {
+	if err := xlMeta.Load(e.metadata); err != nil || len(xlMeta.versions) == 0 {
 		return true
 	}
-	return xlMeta.Versions[len(xlMeta.Versions)-1].Type == DeleteType
+	return xlMeta.versions[0].header.Type == DeleteType
 }
 
 // fileInfo returns the decoded metadata.