ovs-appctl: dpif/show display per bridge stats

ovs-appctl: dpif/show display per bridge stats

This is to fix the fallout of single datapath change.
ovs-appctl dpif/show displays per bridge miss, hit
and flow counts on the screen, but the backend is
obtaining those information from the datapath.
With a single datapath, all bridges of the same
datapath would all display the same  (global)
counters maintained by the datapath, obviously
not correct.

This patch fixes the bug by maintaining per ofproto_dpif
miss and hit counts, which are used for display output.
The number of flows count is obtained by counting the
number facets per ofproto.

ovs-dpctl show still displays the counters maintain by
the datapath, as before.

Bug #15369

Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>