ofproto-dpif-ipfix: add support for per-flow drop counters

ofproto-dpif-ipfix: add support for per-flow drop counters

Patch based on RFC 5102, section 5.10. It implements per-flow drop counters:
- droppedPacketDeltaCount
- droppedPacketTotalCount
- droppedOctetDeltaCount
- droppedOctetTotalCount

In order to determine if packet is going to be dropped, flow actions associated
with packet are read. If at least one of the following conditions is met,
packet is not marked as dropped.

 Packet has at least one:
 - OVS_ACTION_ATTR_OUTPUT action
 - OVS_ACTION_ATTR_CLONE action with nested OVS_ACTION_ATTR_OUTPUT action
 - OVS_ACTION_ATTR_SAMPLE action with nested OVS_ACTION_ATTR_OUTPUT action and
   sampling probability is set to 100%

Signed-off-by: Przemyslaw Szczerbik <przemyslawx.szczerbik@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>