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