Do not pick directory authorities as directory caches
I don't think there should be a reason why dir-auths should get picked by clients as directory caches. There are thousands of other relays being available for that and dir-auths might have some reliability issues once, say, millions of clients show up.
Ideally, we would just be able to set DirCache = 0 at dir-auths but is not possible right now as the DirPort they set is tied to that. So, if we could decouple the DirPort part easily from the DirCache option that would be awesome.
As the second best option we could have a Tor patch which just omits the dir-auths from being picked by clients as dir-caches while leaving everything else in place.