fix: use buffers only when necessary for io.Copy() (#11229)

Use separate sync.Pool for writes/reads

Avoid passing buffers for io.CopyBuffer()
if the writer or reader implement io.WriteTo or io.ReadFrom
respectively then its useless for sync.Pool to allocate
buffers on its own since that will be completely ignored
by the io.CopyBuffer Go implementation.

Improve this wherever we see this to be optimal.

This allows us to be more efficient on memory usage.
```
   385  // copyBuffer is the actual implementation of Copy and CopyBuffer.
   386  // if buf is nil, one is allocated.
   387  func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
   388  	// If the reader has a WriteTo method, use it to do the copy.
   389  	// Avoids an allocation and a copy.
   390  	if wt, ok := src.(WriterTo); ok {
   391  		return wt.WriteTo(dst)
   392  	}
   393  	// Similarly, if the writer has a ReadFrom method, use it to do the copy.
   394  	if rt, ok := dst.(ReaderFrom); ok {
   395  		return rt.ReadFrom(src)
   396  	}
```

From readahead package
```
// WriteTo writes data to w until there's no more data to write or when an error occurs.
// The return value n is the number of bytes written.
// Any error encountered during the write is also returned.
func (a *reader) WriteTo(w io.Writer) (n int64, err error) {
	if a.err != nil {
		return 0, a.err
	}
	n = 0
	for {
		err = a.fill()
		if err != nil {
			return n, err
		}
		n2, err := w.Write(a.cur.buffer())
		a.cur.inc(n2)
		n += int64(n2)
		if err != nil {
			return n, err
		}
```

pkg/ioutil/append-file_nix.go

@@ -40,8 +40,6 @@ func AppendFile(dst string, src string, osync bool) error {
 		return err
 	}
 	defer srcFile.Close()
-	// Allocate staging buffer.
-	var buf = make([]byte, defaultAppendBufferSize)
-	_, err = io.CopyBuffer(appendFile, srcFile, buf)
+	_, err = io.Copy(appendFile, srcFile)
 	return err
 }

pkg/ioutil/append-file_windows.go

@@ -36,8 +36,6 @@ func AppendFile(dst string, src string, osync bool) error {
 		return err
 	}
 	defer srcFile.Close()
-	// Allocate staging buffer.
-	var buf = make([]byte, defaultAppendBufferSize)
-	_, err = io.CopyBuffer(appendFile, srcFile, buf)
+	_, err = io.Copy(appendFile, srcFile)
 	return err
 }

pkg/ioutil/ioutil.go

@@ -22,13 +22,9 @@ import (
 	"io"
 	"os"
 
-	humanize "github.com/dustin/go-humanize"
 	"github.com/minio/minio/pkg/disk"
 )
 
-// defaultAppendBufferSize - Default buffer size for the AppendFile
-const defaultAppendBufferSize = humanize.MiByte
-
 // WriteOnCloser implements io.WriteCloser and always
 // executes at least one write operation if it is closed.
 //
@@ -186,7 +182,7 @@ const directioAlignSize = 4096
 // 4K page boundaries. Without passing aligned buffer may cause
 // this function to return error.
 //
-// This code is similar in spirit to io.CopyBuffer but it is only to be
+// This code is similar in spirit to io.Copy but it is only to be
 // used with DIRECT I/O based file descriptor and it is expected that
 // input writer *os.File not a generic io.Writer. Make sure to have
 // the file opened for writes with syscall.O_DIRECT flag.