slightly more efficient checking of versionUsesKeysDatabase

It's a mvar lookup either way, but I think this way will be slightly more
efficient. And it reduces the number of places where it's checked to 1.

Database/Keys/Handle.hs

@@ -8,6 +8,7 @@
 module Database.Keys.Handle (
 	DbHandle,
 	newDbHandle,
+	unavailableDbHandle,
 	DbState(..),
 	withDbState,
 	flushDbQueue,
@@ -33,6 +34,9 @@ data DbState = DbClosed | DbOpen H.DbQueue | DbUnavailable
 newDbHandle :: IO DbHandle
 newDbHandle = DbHandle <$> newMVar DbClosed
 
+unavailableDbHandle :: IO DbHandle
+unavailableDbHandle = DbHandle <$> newMVar DbUnavailable
+
 -- Runs an action on the state of the handle, which can change its state.
 -- The MVar is empty while the action runs, which blocks other users
 -- of the handle from running.