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-datatypes.go

@@ -101,8 +101,8 @@ type ObjectInfo struct {
 	// IsDir indicates if the object is prefix.
 	IsDir bool
 
-	// Hex encoded md5 checksum of the object.
-	MD5Sum string
+	// Hex encoded unique entity tag of the object.
+	ETag string
 
 	// A standard MIME type describing the format of the object.
 	ContentType string