ipfix: implement flow caching and aggregation in exporter

ipfix: implement flow caching and aggregation in exporter

Implement a per-exporter flow cache with active timeout expiration.
Add columns "cache_active_timeout" and "cache_max_flows" into table
"IPFIX" to configure each cache.

Add per-flow elements "octetDeltaSumOfSquares",
"minimumIpTotalLength", and "maximumIpTotalLength" to replace
"ethernetTotalLength".  Add per-flow element "flowEndReason" to
indicate whether a flow has expired because of an active timeout, the
cache size limit being reached, or the exporter being stopped.

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