Deprecate ReachableDirAddresses and ClientPreferIPv6DirPort
In 0.2.8, we made all client directory fetches use begindir over the ORPort.
This makes ReachableDirAddresses and ClientPreferIPv6DirPort useless for clients. And they don't apply to relays, so we should get rid of them.
We should also check and remove all the reachability code that uses FIREWALL_DIR_CONNECTION. This will help to simplify that code as well.