ofproto-dpif: Factor NetFlow active timeouts out of flow expiration.

ofproto-dpif: Factor NetFlow active timeouts out of flow expiration.

NetFlow active timeouts were only mixed in with flow expiration for
convenience: both processes need to iterate all the facets.  But
an upcoming commit will change flow expiration to work in terms of
a new "subfacet" entity, so they will no longer fit together well.

This change could be seen as an optimization, since NetFlow active
timeouts don't ordinarily have to run as often as flow expiration,
especially when the flow expiration rate is stepped up due to a
large volume of flows.