ofproto: New function ofproto_refresh_rule().

This function checks for a rule in the classifier:
* If the rule exists, reset its modified time.
* If an equivalent rule exists, reset that rule's modified time.
* If no rule exists, re-install the rule and reset its modified time.
* Finally, return the rule that was modified.

This function will be used to ensure that hard timeouts for learnt rules
are refreshed if traffic consistently hits a rule with a learn action in
it. The first user will be the next commit.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>
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v2: Acked.
v1: Ensure rule->modified is updated correctly.
RFC: First post.