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-listobjects_test.go

@@ -549,7 +549,7 @@ func testListObjects(obj ObjectLayer, instanceType string, t TestErrHandler) {
 				if testCase.result.Objects[j].Name != result.Objects[j].Name {
 					t.Errorf("Test %d: %s: Expected object name to be \"%s\", but found \"%s\" instead", i+1, instanceType, testCase.result.Objects[j].Name, result.Objects[j].Name)
 				}
-				if result.Objects[j].MD5Sum == "" {
+				if result.Objects[j].ETag == "" {
 					t.Errorf("Test %d: %s: Expected md5sum to be not empty, but found empty instead", i+1, instanceType)
 				}