Implement IPFIX export

Implement IPFIX export

Define a new NXAST_SAMPLE OpenFlow vendor action and the corresponding
OFPACT_SAMPLE OVS action, to do per-flow packet sampling, translated
into a new SAMPLE "flow_sample" dp action.

Make the userspace action's userdata size vary depending on the union
member used.  Add a new "flow_sample" upcall to do per-flow packet
sampling.  Add a new "ipfix" upcall to do per-bridge packet sampling
to IPFIX collectors.

Extend the OVSDB schema to support configuring IPFIX collector sets.
Add support for configuring multiple IPFIX collectors for per-flow
packet sampling.  Add support for configuring per-bridge IPFIX
sampling.

Automatically generate standard IPFIX entity definitions from the IANA
specs.  Send one IPFIX data record message for every packet sampled by
an OpenFlow sample action or received by a bridge configured with
IPFIX sampling, and periodically send IPFIX template set messages.

Signed-off-by: Romain Lenglet <rlenglet@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>