convert incremental fsck to using sqlite database

Did not keep backwards compat for sticky bit records. An incremental fsck
that is already in progress will start over on upgrade to this version.

This is not yet ready for merging. The autobuilders need to have sqlite
installed.

Also, interrupting a fsck --incremental does not commit the database.
So, resuming with fsck --more restarts from beginning.

Memory: Constant during a fsck of tens of thousands of files.
(But, it does seem to buffer whole transation in memory, so
may really scale with number of files.)

CPU: ?

Database/Types.hs

@@ -0,0 +1,27 @@
+{- types for SQL databases
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# LANGUAGE TemplateHaskell #-}
+
+module Database.Types where
+
+import Database.Persist.TH
+import Data.Maybe
+
+import Types.Key
+
+-- A serialized Key
+newtype SKey = SKey String
+	deriving (Show, Read)
+
+toSKey :: Key -> SKey
+toSKey = SKey . key2file
+
+fromSKey :: SKey -> Key
+fromSKey (SKey s) = fromMaybe (error $ "bad serialied key " ++ s) (file2key s)
+
+derivePersistField "SKey"