internode lockArgs should use messagepack (#13329)

it would seem like using `bufio.Scan()` is very
slow for heavy concurrent I/O, ie. when r.Body
is slow , instead use a proper
binary exchange format, to marshal and unmarshal
the LockArgs datastructure in a cleaner way.

this PR increases performance of the locking
sub-system for tiny repeated read lock requests
on same object.

```
BenchmarkLockArgs
BenchmarkLockArgs-4              6417609               185.7 ns/op            56 B/op          2 allocs/op
BenchmarkLockArgsOld
BenchmarkLockArgsOld-4           1187368              1015 ns/op            4096 B/op          1 allocs/op
```

cmd/lock-rest-server_test.go

@@ -0,0 +1,104 @@
+// 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 (
+	"bufio"
+	"bytes"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"sort"
+	"strconv"
+	"testing"
+
+	"github.com/minio/minio/internal/dsync"
+)
+
+func BenchmarkLockArgs(b *testing.B) {
+	args := dsync.LockArgs{
+		Owner:     "minio",
+		UID:       "uid",
+		Source:    "lockArgs.go",
+		Quorum:    3,
+		Resources: []string{"obj.txt"},
+	}
+
+	argBytes, err := args.MarshalMsg(nil)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	req := &http.Request{
+		Body: ioutil.NopCloser(bytes.NewReader(argBytes)),
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		getLockArgs(req)
+	}
+}
+
+func BenchmarkLockArgsOld(b *testing.B) {
+	values := url.Values{}
+	values.Set("owner", "minio")
+	values.Set("uid", "uid")
+	values.Set("source", "lockArgs.go")
+	values.Set("quorum", "3")
+
+	req := &http.Request{
+		Form: values,
+		Body: ioutil.NopCloser(bytes.NewReader([]byte(`obj.txt`))),
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		getLockArgsOld(req)
+	}
+}
+
+func getLockArgsOld(r *http.Request) (args dsync.LockArgs, err error) {
+	values := r.Form
+	quorum, err := strconv.Atoi(values.Get("quorum"))
+	if err != nil {
+		return args, err
+	}
+
+	args = dsync.LockArgs{
+		Owner:  values.Get("onwer"),
+		UID:    values.Get("uid"),
+		Source: values.Get("source"),
+		Quorum: quorum,
+	}
+
+	var resources []string
+	bio := bufio.NewScanner(r.Body)
+	for bio.Scan() {
+		resources = append(resources, bio.Text())
+	}
+
+	if err := bio.Err(); err != nil {
+		return args, err
+	}
+
+	sort.Strings(resources)
+	args.Resources = resources
+	return args, nil
+}