Always treat series without watermarks as too old.

Current series always get watermarks written out upon append now. This
drops support for old series without any watermarks by always reporting
them as too old (stale) during queries.

storage/metric/tiered.go

@@ -343,7 +343,8 @@ func (t *TieredStorage) seriesTooOld(f *clientmodel.Fingerprint, i time.Time) (b
 			return wmTime.Before(i), nil
 		}
 
-		return false, nil
+		t.wmCache.Set(f, &watermarks{})
+		return true, nil
 	}
 
 	return wm.High.Before(i), nil