pgsql: Make the walwriter close it's handle to an old xlog segment if

Log Message:
-----------
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.

Tags:
----
REL8_4_STABLE

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xlog.c (r1.345.2.8 -> r1.345.2.9)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.345.2.8&r2=1.345.2.9)