Instead of responding with HTTP 503 if image style is already being generated, wait a little bit longer to allow the other process to complete

If another thread is generating the image, we might no longer need the lock to generate the image. We should probably check if the file exists after a timeout. Also, 5 seconds seems quite long. Changed it to 2 seconds.

Hmm, 2 seconds seemed to short after all. Sorry about that. Checking if the file exists again seems like a good idea though. No need to add another lock in that case.

Very rarely we run into this issue as well, so I research a bit and found that meanwhile most browsers do support the `Retry-After` Header, so I suggest to alter the comment. https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Retr...

Firefox has been working on this issue for 22 year :-( https://bugzilla.mozilla.org/show_bug.cgi?id=230260

As this is a temporary problem, I can accept that it appears from time to time, nevertheless it would be nice if this message does not appear to often.

But even when retrying it once, it the error could occur when the server is busy. May be stage_files_proxy solution to make several tries is more appropriate:

https://git.drupalcode.org/project/stage_file_proxy/-/blob/3.1.x/src/Con...