rename/unlink handling for Win32

Here is my approach to the use of rename/unlink on Win32. The full
discussion is at:

http://momjian.postgresql.org/cgi-bin/pgtodo?win32

Basically, rename/unlink will fail if the file is opened. You can move
the open file to another name, but that then requires open to loop in
case the file is missing.

The following patch loops over rename/unlink every 1/10th of second,
printing a warning message after 1 second, and printing a completion
message if a warning message was printed.

I looked at PeerDirect's and SRA's port, and neither provides a better
method. I looked at PeerDirect's and it actually has some conditional
code for rename. For example, it has a signal that is sent to all
backends to inform them to close their open WAL files. I am not sure if
that is required for us because there is some replication stuff in
there that we aren't using. Jan? In a few other places, it allows the
rename to fail.

I am inclined to implement it as shown, then see what delayed
rename/unlinks we get in testing.

--
Bruce Momjian | http://candle.pha.pa.us
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