ofproto-dpif: Keep track of exact-match flow info

This patch adds more flow related stats to the output of
"ovs-appctl dpif/show".  Specifically, the follow information
are added per ofproto:

- Max flow table size
- Average flow table size
- Average flow table add rate
- Average flow table delete rate
- Average flow entry life in milliseconds

Feature #15366

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