pktout: Use SDN_Types.pktOut type for packet outs

Additionaly, the controller must keep track of the switch that the
packet_out is intended for.
pktout: use payload in PacketIn

Reuse types as much as possible rather than redefining them in different
ways across libraries.
Merge pull request #290 from frenetic-lang/pktout

Reuse types from ocaml-openflow in app API