Stop caching extra-info documents on directory mirrors
I suggest stopping to cache extra-info documents on directory mirrors.
Most clients don't need extra-info documents for their normal operation. The only use case that comes to mind is external tools configuring a local tor client to provide them with extra-info documents. Note that we wouldn't break these tools, because they could still fetch from the directory authorities. Though I assume there are very few requests for extra-info documents in the network.
There are also not many directory mirrors in the network that serve extra-info documents: in the first weeks of April 2014, only 0.46% of server descriptors contained the "caches-extra-info" line, so 0.46% * 5,000 = 23 relays. Note that 9 of these are directory authorities and only 14 are directory mirrors.
But still, these 14 directory mirrors need to keep their cache up-to-date by fetching extra-info documents from the directory authorities. If there are fewer than 14 clients downloading extra-info documents from the caches, the caches put even more load on directory authorities just to keep their caches recent.
All in all, I'd say let's get rid of extra-info documents on directory mirrors. I can write a patch, unless somebody else enjoys removing (mostly) unused code as much as I do.