RE: Random pg_upgrade test failure on drongo

Dear Alexander, 

> I agree with your analysis and would like to propose a PoC fix (see
> attached). With this patch applied, 20 iterations succeeded for me.

There are no reviewers so that I will review again. Let's move the PoC
to the concrete patch. Note that I only focused on fixes of random failure,
other parts are out-of-scope.

Basically, code comments can be updated accordingly.

01.

```
	/*
	 * This function might be called for a regular file or for a junction
	 * point (which we use to emulate symlinks).  The latter must be unlinked
	 * with rmdir() on Windows.  Before we worry about any of that, let's see
	 * if we can unlink directly, since that's expected to be the most common
	 * case.
	 */
	snprintf(tmppath, sizeof(tmppath), "%s.tmp", path);
	if (pgrename(path, tmppath) == 0)
	{
		if (unlink(tmppath) == 0)
			return 0;
		curpath = tmppath;
	}
```

You can modify comments atop changes because it is not trivial.
Below is my draft:

```
	 * XXX: we rename the target file to ".tmp" before calling unlink. The
	 * removal may fail with STATUS_DELETE_PENDING status on Windows, so
	 * creating the same file would fail. This assumes that renaming is a
	 * synchronous operation.
```

02.

```
	loops = 0;
	while (lstat(curpath, &st) < 0 && lstat_error_was_status_delete_pending())
	{
		if (++loops > 100)		/* time out after 10 sec */
			return -1;
		pg_usleep(100000);		/* us */
	}
```

Comments can be added atop the part. Below one is my draft.

```
	/*
	 * Wait until the removal is really finished to avoid ERRORs for creating a
	 * same file in other functions.
	 */
```


Best Regards,
Hayato Kuroda
FUJITSU LIMITED