openflow.nicira: Use single OF handler instance
openflow.nicira: Use single OF handler instance

Rather than creating a new instance of NiciraOpenFlowHandlers for
every connection, we now just create one and reuse it.  This avoids
the relatively expensive initialization process for each connection.
(We still pay this price for the handshake handlers, unfortunately,
but at least we don't pay it twice.)

MurphyMc/pox,MurphyMc/pox,pthien92/sdn,noxrepo/pox,noxrepo/pox,MurphyMc/pox,pthien92/sdn,noxrepo/pox,MurphyMc/pox,pthien92/sdn,pthien92/sdn,MurphyMc/pox,pthien92/sdn,noxrepo/pox