Reject all relays and bridges running 0.2.0.x

If we are going to start enforcing proposal 110's requirement that EXTEND relay commands only be sent inside RELAY_EARLY cells (legacy/trac#4339 (moved)), we must remove all servers (both relays and bridges) running Tor 0.2.0.x from the network.

This needs to be a dirauth-only change; clients and non-dirauth servers must still accept and mirror descriptors for 0.2.0.x servers.