Make all relays automatically be dir caches

During the entry guard discussions, we have decided that it's a good idea to make all relays directory servers. We mainly needed the entry guards to be directories, but it seems easier and more elegant to just turn all relays to directory servers.

This is easier nowadays than in the past because BEGIN_DIR makes it so that directory servers don't need to have a separate DirPort open. (However, maybe relays get the V2Dir flag only if they have a DirPort open?)

Also, since all relays have all the directory documents anyway, it doesn't further bloat their disk to become directory servers.