Caching for mini panels

Here's something to get started. The first time it executes per page load it checks if result set was cached before and if so then just return that data; if nothing was cached previously, it compiles the mini_panels list per normal then saves the results to the cache and in a static variable. Subsequent function calls will have valid data in the static variable so just returns that.

FYI this code was originally written by datacalibre, I just turned it into a patch.

Forgot to update the status.

It's purely a performance thing, Datacaliber did some profiling on skinet.com prior to launch and found this patch shaved a few hundred ms off the page load. This works well with the caching engine (Panels plugin! Coming soon) that he built which gets logged-in page load on complex pages down really low.

This needs additional work as there's currently no provision for flushing the cache after modifications are made.

Amazing stuff! Looking forward to the next release!