ofproto-dpif: Consolidate facet stat logic.

ofproto-dpif: Consolidate facet stat logic.

The logic for updating statistics at the facet level had been
spread through ofproto-dpif in a rather confusing manner.  This
patch consolidates as much of this logic as is reasonable into
facet_push_stats().

On a side note, I'd expect this patch to have a marginal positive
performance impact when using learning (though I haven't bothered
to measure it).  It combines facet_learn() and facet_push_stats()
into one step allowing us to avoid a redundant xlate_actions().

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