ofproto: per-table statistics

Add per-table counters. This resolves some short-comings
in the data provided in a table stats reply message.

* Lookups and matches are calculated based on table
  accesses rather than datapath flow hits and misses.

* Lookups and matches are credited to the table where they
  occurred rather than all being credited to table 0.

These problems were observed when running make check-ryu
and this patch allows many of its tester.py match checks
to pass.

Reviewed-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>