eliminate single/multi writer distinction

After commit f4bdecc4ec, there is no
longer any distinction between SingleWriter and MultiWriter's handling
of read after write.

Databases that were SingleWriter still have lock files that are used to
prevent multiple writers.

This does make writing to such databases a bit more expensive,
because the MultiWriter code path that is now used opens a second db
connection in order to write to them.

Database/Export.hs

@@ -104,7 +104,7 @@ openDb u = do
 	unlessM (liftIO $ R.doesPathExist db) $ do
 		initDb db $ void $
 			runMigrationSilent migrateExport
-	h <- liftIO $ H.openDbQueue H.SingleWriter db "exported"
+	h <- liftIO $ H.openDbQueue db "exported"
 	return $ ExportHandle h u
 
 closeDb :: ExportHandle -> Annex ()