ofctl_rest: support OFPFlowStats filtered by fields

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

usage)

  URI:    /stats/flow/<dpid>
  method: POST

  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.

e.g.)

  curl -X POST -d '{"table_id": 0,
                    "out_port": 2,
                    "cookie": 1,
                    "cookie_mask": 1,
                    "match":{"in_port":1}}'
  http://localhost:8080/stats/flow/1

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>