ofproto-dpif: Push statistics less frequently.

The most natural place to push facet statistics is in
update_stats() where they're pulled from the datapath.  However,
under load, update_stats() can be called as many as 10 times per
second causing us to push statistics so frequently it hurts
performance.  By pushing statistics much less frequently, this
patch generates a roughly 8% improvement in TCP_CRR performance.

Signed-off-by: Ethan Jackson <ethan@nicira.com>