Caching decorator with timeout invalidation

This script is a caching decorator that  collects garbage in a separate thread (for performance).

It allows each cached function to (optionally) set a custom maximum age for entries, and allows individual cache entries to be selectiries, andlidated.

This kind of caching decorator is often useful in web development, when your pages are dynamic enough that you can't cache the whole page, but would still like to cache individual functions that are used to build the page.

The selective invalidation is typically used when some action occurs that makes some of the cache results invalid; for example, if you're displaying message board posts, and a user posts a new message, you would want to invalidate the cache for a function that finds the most recent posts.