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/xl-storage-format-v2_string.go

@@ -0,0 +1,27 @@
+// Code generated by "stringer -type VersionType -output=xl-storage-format-v2_string.go xl-storage-format-v2.go"; DO NOT EDIT.
+
+package cmd
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[invalidVersionType-0]
+	_ = x[ObjectType-1]
+	_ = x[DeleteType-2]
+	_ = x[LegacyType-3]
+	_ = x[lastVersionType-4]
+}
+
+const _VersionType_name = "invalidVersionTypeObjectTypeDeleteTypeLegacyTypelastVersionType"
+
+var _VersionType_index = [...]uint8{0, 18, 28, 38, 48, 63}
+
+func (i VersionType) String() string {
+	if i >= VersionType(len(_VersionType_index)-1) {
+		return "VersionType(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _VersionType_name[_VersionType_index[i]:_VersionType_index[i+1]]
+}