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-legacy.go

@@ -0,0 +1,89 @@
+// Copyright (c) 2015-2021 MinIO, Inc.
+//
+// This file is part of MinIO Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/tinylib/msgp/msgp"
+)
+
+// unmarshalV unmarshals with a specific header version.
+func (x *xlMetaV2VersionHeader) unmarshalV(v uint8, bts []byte) (o []byte, err error) {
+	switch v {
+	case 1:
+		return x.unmarshalV1(bts)
+	case xlHeaderVersion:
+		return x.UnmarshalMsg(bts)
+	}
+	return bts, fmt.Errorf("unknown xlHeaderVersion: %d", v)
+}
+
+// unmarshalV1 decodes version 1, never released.
+func (x *xlMetaV2VersionHeader) unmarshalV1(bts []byte) (o []byte, err error) {
+	var zb0001 uint32
+	zb0001, bts, err = msgp.ReadArrayHeaderBytes(bts)
+	if err != nil {
+		err = msgp.WrapError(err)
+		return
+	}
+	if zb0001 != 4 {
+		err = msgp.ArrayError{Wanted: 4, Got: zb0001}
+		return
+	}
+	bts, err = msgp.ReadExactBytes(bts, (x.VersionID)[:])
+	if err != nil {
+		err = msgp.WrapError(err, "VersionID")
+		return
+	}
+	x.ModTime, bts, err = msgp.ReadInt64Bytes(bts)
+	if err != nil {
+		err = msgp.WrapError(err, "ModTime")
+		return
+	}
+	{
+		var zb0002 uint8
+		zb0002, bts, err = msgp.ReadUint8Bytes(bts)
+		if err != nil {
+			err = msgp.WrapError(err, "Type")
+			return
+		}
+		x.Type = VersionType(zb0002)
+	}
+	{
+		var zb0003 uint8
+		zb0003, bts, err = msgp.ReadUint8Bytes(bts)
+		if err != nil {
+			err = msgp.WrapError(err, "Flags")
+			return
+		}
+		x.Flags = xlFlags(zb0003)
+	}
+	o = bts
+	return
+}
+
+// unmarshalV unmarshals with a specific metadata version.
+func (j *xlMetaV2Version) unmarshalV(v uint8, bts []byte) (o []byte, err error) {
+	switch v {
+	// We accept un-set as latest version.
+	case 0, xlMetaVersion:
+		return j.UnmarshalMsg(bts)
+	}
+	return bts, fmt.Errorf("unknown xlMetaVersion: %d", v)
+}