ofproto-dpif: Push statistics in rule_get_stats().

ofproto-dpif: Push statistics in rule_get_stats().

As time goes on, and flow tables become more complicated, the
tradeoff between keeping up to date statistics, and the CPU
resources needed to maintain them, will become more important.
Commit 5c0243a (ofproto-dpif: xlate actions once with subfacets.)
delayed the reporting of some statistics in an effort to achieve
higher flow setup performance.  Future commits will continue in the
same direction.

This patch helps to alleviate the issue, by pushing statistics
rule_get_stats(), when users actually want them.  Presumably, this
happens rarely, and thus will not have a negative impact on
ovs-vswitchd performance.

Signed-off-by: Ethan Jackson <ethan@nicira.com>