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-utils.go

@@ -187,6 +187,35 @@ func getCompleteMultipartMD5(parts []completePart) (string, error) {
 	return s3MD5, nil
 }
 
+// Clean meta etag keys 'md5Sum', 'etag'.
+func cleanMetaETag(metadata map[string]string) map[string]string {
+	return cleanMetadata(metadata, "md5Sum", "etag")
+}
+
+// Clean metadata takes keys to be filtered
+// and returns a new map with the keys filtered.
+func cleanMetadata(metadata map[string]string, keyNames ...string) map[string]string {
+	var newMeta = make(map[string]string)
+	for k, v := range metadata {
+		if contains(keyNames, k) {
+			continue
+		}
+		newMeta[k] = v
+	}
+	return newMeta
+}
+
+// Extracts etag value from the metadata.
+func extractETag(metadata map[string]string) string {
+	// md5Sum tag is kept for backward compatibility.
+	etag, ok := metadata["md5Sum"]
+	if !ok {
+		etag = metadata["etag"]
+	}
+	// Success.
+	return etag
+}
+
 // Prefix matcher string matches prefix in a platform specific way.
 // For example on windows since its case insensitive we are supposed
 // to do case insensitive checks.