fs/erasure: Rename meta 'md5Sum' as 'etag'. (#4319)

This PR also does backend format change to 1.0.1
from 1.0.0.  Backward compatible changes are still
kept to read the 'md5Sum' key. But all new objects
will be stored with the same details under 'etag'.

Fixes #4312

cmd/object-api-common.go

@@ -35,8 +35,8 @@ const (
 	// Objects meta prefix.
 	objectMetaPrefix = "objects"
 
-	// Md5Sum of empty string.
-	emptyStrMd5Sum = "d41d8cd98f00b204e9800998ecf8427e"
+	// ETag (hex encoded md5sum) of empty string.
+	emptyETag = "d41d8cd98f00b204e9800998ecf8427e"
 )
 
 // Global object layer mutex, used for safely updating object layer.
@@ -68,10 +68,10 @@ func dirObjectInfo(bucket, object string, size int64, metadata map[string]string
 	// This is a special case with size as '0' and object ends with
 	// a slash separator, we treat it like a valid operation and
 	// return success.
-	md5Sum := metadata["md5Sum"]
-	delete(metadata, "md5Sum")
-	if md5Sum == "" {
-		md5Sum = emptyStrMd5Sum
+	etag := metadata["etag"]
+	delete(metadata, "etag")
+	if etag == "" {
+		etag = emptyETag
 	}
 
 	return ObjectInfo{
@@ -81,7 +81,7 @@ func dirObjectInfo(bucket, object string, size int64, metadata map[string]string
 		ContentType: "application/octet-stream",
 		IsDir:       true,
 		Size:        size,
-		MD5Sum:      md5Sum,
+		ETag:        etag,
 		UserDefined: metadata,
 	}
 }