ofproto-dpif: Move special upcall handling into ofproto-dpif-upcall.

Both the IPFIX and SFLOW modules are thread safe, so there's no
particular reason to pass them up to the main thread.  Eliminating
this step significantly simplifies the code.

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