use WAL mode to ensure read from db always works, even when it's being written to

Also, moved the database to a subdir, as there are multiple files.

This seems to work well with concurrent fscks, although they still do
redundant work due to the commit granularity. Occasionally two writes will
conflict, and one is then deferred and happens later.

Except, with 3 concurrent fscks, I got failures:

git-annex: user error (SQLite3 returned ErrorBusy while attempting to perform prepare "SELECT \"fscked\".\"key\"\nFROM \"fscked\"\nWHERE \"fscked\".\"key\" = ?\n": database is locked)

Argh!!!

Database/Fsck.hs

@@ -21,7 +21,8 @@ module Database.Fsck (
 import Database.Types
 import qualified Database.Handle as H
 import Locations
-import Utility.Directory
+import Utility.PosixFiles
+import Utility.Exception
 import Annex
 import Types.Key
 import Types.UUID
@@ -34,6 +35,7 @@ import Control.Monad
 import Control.Monad.IfElse
 import Control.Monad.IO.Class (liftIO)
 import System.Directory
+import System.FilePath
 import Data.Maybe
 import Control.Applicative
 
@@ -56,20 +58,27 @@ Fscked
 newPass :: UUID -> Annex Bool
 newPass u = isJust <$> tryExclusiveLock (gitAnnexFsckDbLock u) go
   where
-	go = liftIO. nukeFile =<< fromRepo (gitAnnexFsckDb u)
+	go = liftIO . void . tryIO . removeDirectoryRecursive
+		=<< fromRepo (gitAnnexFsckDbDir u)
 
 {- Opens the database, creating it atomically if it doesn't exist yet. -}
 openDb :: UUID -> Annex FsckHandle
 openDb u = do
-	db <- fromRepo (gitAnnexFsckDb u)
+	dbdir <- fromRepo (gitAnnexFsckDbDir u)
+	let db = dbdir </> "db"
 	unlessM (liftIO $ doesFileExist db) $ do
-		let newdb = db ++ ".new"
-		h <- liftIO $ H.openDb newdb
-		void $ liftIO $ H.commitDb h $
-			void $ runMigrationSilent migrateFsck
-		liftIO $ H.closeDb h
-		setAnnexFilePerm newdb
-		liftIO $ renameFile newdb db
+		let tmpdbdir = dbdir ++ ".tmp"
+		let tmpdb = tmpdbdir </> "db"
+		liftIO $ do
+			createDirectoryIfMissing True tmpdbdir
+			h <- H.initDb tmpdb $ void $
+				runMigrationSilent migrateFsck
+			H.closeDb h
+		setAnnexDirPerm tmpdbdir
+		setAnnexFilePerm tmpdb
+		liftIO $ do
+			void $ tryIO $ removeDirectoryRecursive dbdir
+			rename tmpdbdir dbdir
 	lockFileShared =<< fromRepo (gitAnnexFsckDbLock u)
 	h <- liftIO $ H.openDb db
 	return $ FsckHandle h u