PATCH speedup object creation for STI queries... Could this get in before 3.0.0final?

Hey,

my app heavily depends on STI and without this patch my dashboard
takes > 3000ms with the patch applies < 300ms (still too slow, I
know). As i'm just adding "memoize :find_sti_class" to AR::Base I
believe the patch should be safe - but maybe I'm wrong as this will
reference the class constants and the class unloading in dev-mode
might break?

Also I have not written a test as I'm not too sure what to test - If
you come up with a clever way of testing this let me know and I'll
write that test!

ticket is:
https://rails.lighthouseapp.com/projects/8994/tickets/5403

re,
thies