Handle flat permalink structure with more than 32K posts

As mentioned in the help pages:
> ### How many pages can I cache? ###
> The only real limit are limits defined by your server. For example, EXT2 and EXT3 allow a maximum of 31,999 sub directories so if you have a flat permalink structure (like /%POSTNAME%/) and more than 32,000 posts you may run into problems. Likewise, if you run a multisite network and have more than 31,999 sites (blogs) you won't be able to cache all of them. Realistically if you had that many active sites you wouldn't be running on one server.

I see that one of my sites might reach that limit in a couple of months, and would like to ask to not be forced to change my flat permalink structure. Would it be possible to have an option for wp-super-cache to use sub folders (say /year/post-name in the cached folder) without forcing me to change my permalink structure?

No source file, test, or entry point is named. Start by tracing how wp-super-cache chooses directories for cached flat permalinks, then determine where an optional nested cache path could be configured without changing the public permalink; done means supporting sites with more than 32,000 posts while preserving existing URLs and cache behavior.

Written by the indexing model from the issue text.