XL: Implement new ReadAll API for files which are read in single call. (#1974)

Add a unit test as well.

storage-interface.go

@@ -31,4 +31,7 @@ type StorageAPI interface {
 	RenameFile(srcVolume, srcPath, dstVolume, dstPath string) error
 	StatFile(volume string, path string) (file FileInfo, err error)
 	DeleteFile(volume string, path string) (err error)
+
+	// Read all.
+	ReadAll(volume string, path string) (buf []byte, err error)
 }