Change Windows rename and unlink substitutes so that they time out after

Change Windows rename and unlink substitutes so that they time out after
30 seconds instead of retrying forever.  Also modify xlog.c so that if
it fails to rename an old xlog segment up to a future slot, it will
unlink the segment instead.  Per discussion of bug #2712, in which it
became apparent that Windows can handle unlinking a file that's being
held open, but not renaming it.