Extract hooks also into their non-WAL location

The TransactionManager is storing new versions of custom hooks always into a new directory. This ensures we can isolate transactions reading custom hooks from concurrent updates. This creates a problem if we want the WAL logic to be safe to toggle off without losing data. If the hooks are only written to the new location, they won't be used anymore if the WAL logic is subsequently disabled. To make the WAL safe to toggle on and off, this commit writes the custom hooks from the log also to their old location in <repo>/custom_hooks. This ensures that the hook updates remain in place even if we have to disable the WAL after enabling it for some time.

Part of #5058 (closed)