Race condition in _less_pre_render()

As it's currently written, there's a race condition in _less_pre_render() in the event of high concurrency.

If you flush the cache, and the site is simultaneously serving a large number of pages, some of those pages will get no styles, and you'll see log messages like

LESS could not create a directory in public://less/5139043775cd21.37298789/sites/all/themes/themename/css

This appears to be because the process of creating the deep directory structure can take a few ms, and if two start doing it at once there can be issues.

I'm seeing this in part because our system uses network-attached storage which stretches out the time to create the directories in such a way that this happens a non-trivial number of times.

Alternative patterns might be:

• writing to a single file with a unique name
• adding some kind of locking while generating the aggregate (ala image_style_deliver())
• adding a retry

I'd argue in favor of using single files rather than a directory tree as it makes cleaning up a lot easier too.

I'm not sure how much this use-case matters to you, but if you're open to this proposal I can roll a patch.