reactive: fix bug in OF0x01 features request

Once the features stage receives the features response, it does not
remove the client from the list of clients from which it is awaiting
features.
reactive: Make PortStatusMeg.to_string more verbose

More details for better debuggability of the reactive controller.

Related to #79
reactive: fix parse function for PortState
of0x01: Rework repesentation of stp_state (Closes #82)

ofp_port_state carries both a flag (whether or not the link is down) as
well as an enumeration (how the port is involved in a spanning tree
algorithm). The spanning tree states are mutually exclusive, so the code
now represents the possible states as an enumeration rather than a
collection of booleans.
of0x01: Fix bug in PortStatus parsing (Closes #81)

The code was shifting the Cstruct buffer and then throwing away the
shifted buffer, and then parsing PortDescription from the beginning of
the PortStatus message. The code now uses the shifted Cstruct buffer to
parse the PortDescription.
of0x01: Roundtrip testing for Port-related messages