ofproto: Add support for flow cookies (OpenFlow 1.0)

In OpenFlow 1.0, flows have been extended to include an opaque
identifier, referred to as a cookie. The cookie is specified by the
controller when the flow is installed; the cookie will be returned as
part of each flow stats and flow removed message.

NOTE: OVS at this point is not wire-compatible with OpenFlow 1.0 until
the final commit in this Openflow 1.0 set.