better approach to enabling WAL mode

The old approach opened the database an extra time to enable WAL mode,
but more recent persistent-sqlite has a better API to enable it.

Database/Handle.hs

@@ -1,6 +1,6 @@
 {- Persistent sqlite database handles.
  -
- - Copyright 2015 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}