ofctl_rest: support OFPFlowStats filtered by fields (Extended)

this patch enables ofctl_rest to get OFPFlowStats messages
filtered by the OFPFlowStatsRequest fields in OpenFlow specification.

usage)

  URI:    /stats/flow/<dpid>
  method: POST (or GET)

  the message body is as follows:

  table_id     ID of table.
  out_port     Require matching entries to include this as an output port.
  out_group    Require matching entries to include this as an output group.
               (Not supported in of1.0)
  cookie       Require matching entries to contain this cookie value.
               (Not supported in of1.0)
  cookie_mask  Mask used to restrict the cookie bits that must match.
               (Not supported in of1.0)
  match        Fields to match.
  priority     Priority of the entry.
               (Not in OFPFlowStatsRequest fields)
  actions      Actions.
               (Not in OFPFlowStatsRequest fields)

e.g.)

  curl -X POST -d '{"table_id": 0,
                    "out_port": 2,
                    "cookie": 1,
                    "cookie_mask": 1,
                    "match":{"in_port":1},
                    "priority": 11111,
                    "actions":[{"port":1, "type":"OUTPUT"}]}'
  http://localhost:8080/stats/flow/1

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>