Refactor port registration on dp connect

This prepares the code for a later change over to OF1.3.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Switch to OpenFlow 1.3

OpenFlow 1.3 no longer supplies a list of ports as part of the
initial connection, so we send a stats request to gather this
information and handle the replies.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Support Ryu 3.2

Signed-off-by: Joe Stringer <joe@wand.net.nz>