ofproto: Increase default datapath max_idle time.

The datapath max_idle value determines how long to wait before deleting
an idle datapath flow when operating below the flow_limit. This patch
increases the max_idle to 10 seconds, which allows datapath flows to be
remain cached even if they are used less consistently, and provides a
small improvement in the supported number of flows when operating around
the flow_limit.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>