- actively remove entries from the cache when they are found to be stale. This removes the possible causes of both the KeyError bug that Brian patched in Dec 2000 and the KeyError bug reported in #2212
- remove arbitary "divide by 2" on when we start checking to empty the cache and tcache, it looks like this may have once made sense when the tcache could grow without the cache growing, but I still can't really see how it made sense :-S
- fix of-by-one errors on cache size