Load limiting work for Recentchanges RSS/Atom feeds:

Load limiting work for Recentchanges RSS/Atom feeds:
* Cache feeds for up to a day
* Maximum number of rows to show
* Minimum timeout between changes to re-cache
* Maximum article size to show diffs for (will show diff link instead for larger)
* Also, return 500 error if asked for an invalid feed format instead of silently showing the html