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
```

internal/dsync/lock-args.go

@@ -0,0 +1,40 @@
+// 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 dsync
+
+//go:generate msgp -file $GOFILE
+
+// LockArgs is minimal required values for any dsync compatible lock operation.
+type LockArgs struct {
+	// Unique ID of lock/unlock request.
+	UID string
+
+	// Resources contains single or multiple entries to be locked/unlocked.
+	Resources []string
+
+	// Source contains the line number, function and file name of the code
+	// on the client node that requested the lock.
+	Source string
+
+	// Owner represents unique ID for this instance, an owner who originally requested
+	// the locked resource, useful primarily in figuring our stale locks.
+	Owner string
+
+	// Quorum represents the expected quorum for this lock type.
+	Quorum int
+}