ofproto-dpif: Compute the subfacet add/del rate using coverage counters.

ofproto-dpif: Compute the subfacet add/del rate using coverage counters.

So far, the subfacet rates (e.g. add rate, del rate) are computed by
exponential moving averaging function in ofproto-dpif.c.  This commit
replaces that logic with coverage counters.  And the rates can be
checked by running "ovs-appctl coverage/show" command.

Signed-off-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>