Make the walwriter close it's handle to an old xlog segment if it's no longer

Make the walwriter close it's handle to an old xlog segment if it's no longer
the current one. Not doing this would leave the walwriter with a handle to a
deleted file if there was nothing for it to do for a long period of time,
preventing the file from  being completely removed.

Reported by Tollef Fog Heen, and thanks to Heikki for some hand-holding with
the patch.