Optimize FileInfo(Version) transfer (#10775)

File Info decoding, in particular, is showing up as a major 
allocator and time consumer for internode data transfers

Switch to message pack for cross-server transfers:

```
MSGP:

Size: 945 bytes

BenchmarkEncodeFileInfoMsgp-32    	 1558444	       866 ns/op	   1.16 MB/s	       0 B/op	       0 allocs/op
BenchmarkDecodeFileInfoMsgp-32    	  479968	      2487 ns/op	   0.40 MB/s	     848 B/op	      18 allocs/op

GOB:

Size: 1409 bytes

BenchmarkEncodeFileInfoGOB-32    	  333339	      3237 ns/op	   0.31 MB/s	     576 B/op	      19 allocs/op
BenchmarkDecodeFileInfoGOB-32    	   20869	     57837 ns/op	   0.02 MB/s	   16439 B/op	     428 allocs/op
```

cmd/storage-datatypes.go

@@ -17,10 +17,11 @@
 package cmd
 
 import (
-	"os"
 	"time"
 )
 
+//go:generate msgp -file=$GOFILE
+
 // VolInfo - represents volume stat information.
 type VolInfo struct {
 	// Name of the volume.
@@ -74,6 +75,8 @@ func (f *FileInfoVersions) forwardPastVersion(v string) {
 }
 
 // FileInfo - represents file stat information.
+//msgp:tuple FileInfo
+// The above means that any added/deleted fields are incompatible.
 type FileInfo struct {
 	// Name of the volume.
 	Volume string
@@ -105,7 +108,7 @@ type FileInfo struct {
 	Size int64
 
 	// File mode bits.
-	Mode os.FileMode
+	Mode uint32
 
 	// File metadata
 	Metadata map[string]string