arp_responder: Fix bugs with re-learning entries

This fixes an exception when attempting to learn an address that
already has an entry using the special "use-DPID-as-MAC" option.
It also looks like learned entries used to be able to replace static
entries, which probably isn't desirable, so this behavior has been
altered.
arp_responder: Remove _dpid_to_mac() function

The functionality in _dpid_to_mac() is now available as the .eth_addr
attribute of the Connection object.  This commit removes the former and
substitutes the latter.