ofproto-dpif: More responsive average subfacet statistics.

ofproto-dpif: More responsive average subfacet statistics.

Before this patch, statistics about subfacet average life span,
and count in the datapath, were calculated over the entire lifetime
of the ofproto.  Furthermore, the subfacet lifespan was only
updated when a subfacet was removed from the datapath causing long
lived subfacets to be ignored.  It's far more useful to know these
numbers averaged over all subfacets in the recent past.  This patch
changes the code to implement an exponentially weighted moving
average updated every time statistics are pulled from the datapath.

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