ofproto: Modularize netflow.

ofproto: Modularize netflow.

The netflow code has its tentacles all over the ofproto-dpif module.
This is fine today, but in future facets, which correspond roughly to
netflow_flows, will be retired.  In preparation, this patch hides as
much implementation detail as possible inside the netflow module.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>