Block change to 1M from 128KiB and extensively use buffer pools.

Following are the benefits of this change.
```
benchmark                           old allocs     new allocs     delta
BenchmarkPutObjectPart5MbFS-4       46172          16383          -64.52%
BenchmarkPutObjectPart10MbFS-4      36849          7223           -80.40%
BenchmarkPutObjectPart25MbFS-4      31699          3865           -87.81%
BenchmarkPutObjectPart50MbFS-4      27577          2481           -91.00%
BenchmarkPutObjectVerySmallFS-4     57             56             -1.75%
BenchmarkPutObject10KbFS-4          57             56             -1.75%
BenchmarkPutObject100KbFS-4         57             56             -1.75%
BenchmarkPutObject1MbFS-4           176            56             -68.18%
BenchmarkPutObject5MbFS-4           720            90             -87.50%
BenchmarkPutObject10MbFS-4          1400           124            -91.14%
BenchmarkPutObject25MbFS-4          3440           261            -92.41%
BenchmarkPutObject50MbFS-4          6841           466            -93.19%
```

cmd/fs-v1.go

@@ -25,6 +25,7 @@ import (
 	"path"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/minio/minio/pkg/disk"
 	"github.com/minio/minio/pkg/mimedb"
@@ -227,6 +228,13 @@ func (fs fsObjects) DeleteBucket(bucket string) error {
 
 /// Object Operations
 
+var getBufPool = sync.Pool{
+	New: func() interface{} {
+		b := make([]byte, readSizeV1)
+		return &b
+	},
+}
+
 // GetObject - get an object.
 func (fs fsObjects) GetObject(bucket, object string, offset int64, length int64, writer io.Writer) (err error) {
 	// Verify if bucket is valid.
@@ -266,8 +274,11 @@ func (fs fsObjects) GetObject(bucket, object string, offset int64, length int64,
 	if length > 0 && bufSize > length {
 		bufSize = length
 	}
+
 	// Allocate a staging buffer.
-	buf := make([]byte, int(bufSize))
+	bufp := getBufPool.Get().(*[]byte)
+	buf := *bufp
+	defer getBufPool.Put(bufp)
 	for {
 		// Figure out the right size for the buffer.
 		curLeft := bufSize
@@ -356,7 +367,15 @@ func (fs fsObjects) GetObjectInfo(bucket, object string) (ObjectInfo, error) {
 	}, nil
 }
 
-// PutObject - create an object.
+var putBufPool = sync.Pool{
+	New: func() interface{} {
+		b := make([]byte, readSizeV1)
+		return &b
+	},
+}
+
+// PutObject - saves an object atomically of size - size bytes.
+// With size bytes of '-1' this function reads till EOF.
 func (fs fsObjects) PutObject(bucket string, object string, size int64, data io.Reader, metadata map[string]string) (string, error) {
 	// Verify if bucket is valid.
 	if !IsValidBucketName(bucket) {
@@ -405,9 +424,12 @@ func (fs fsObjects) PutObject(bucket string, object string, size int64, data io.
 		if size > 0 && bufSize > size {
 			bufSize = size
 		}
-		buf := make([]byte, int(bufSize))
+		bufp := putBufPool.Get().(*[]byte)
+		buf := *bufp
+		defer putBufPool.Put(bufp)
+
 		teeReader := io.TeeReader(limitDataReader, md5Writer)
-		bytesWritten, err := fsCreateFile(fs.storage, teeReader, buf, minioMetaBucket, tempObj)
+		bytesWritten, err := fsCreateFile(fs.storage, teeReader, buf[:bufSize], minioMetaBucket, tempObj)
 		if err != nil {
 			fs.storage.DeleteFile(minioMetaBucket, tempObj)
 			return "", toObjectErr(err, bucket, object)