add key stability checking interface

Needed for resuming from chunks.

Url keys are considered not stable. I considered treating url keys with a
known size as stable, but just don't feel that is enough information.

Backend/WORM.hs

@@ -23,6 +23,7 @@ backend = Backend
 	, fsckKey = Nothing
 	, canUpgradeKey = Nothing
 	, fastMigrate = Nothing
+	, isStableKey = const True
 	}
 
 {- The key includes the file size, modification time, and the