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/Hash.hs

@@ -45,6 +45,7 @@ genBackend hash = Just Backend
 	, fsckKey = Just $ checkKeyChecksum hash
 	, canUpgradeKey = Just needsUpgrade
 	, fastMigrate = Just trivialMigrate
+	, isStableKey = const True
 	}
 
 genBackendE :: Hash -> Maybe Backend

Backend/URL.hs

@@ -25,6 +25,9 @@ backend = Backend
 	, fsckKey = Nothing
 	, canUpgradeKey = Nothing
 	, fastMigrate = Nothing
+	-- The content of an url can change at any time, so URL keys are
+	-- not stable.
+	, isStableKey = const False
 	}
 
 {- Every unique url has a corresponding key. -}

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