ofproto-dpif-upcall: Hardcode max_idle to 1500ms.

Before this patch, OVS tried to guess an optimal max idle time for
datapath flows based on the number of datapath flows relative to the
limit.  This caused instability because the limit was based on the
dump duration which was affected by the max idle time.  This patch
chooses instead to hardcode the max idle time to 1.5s except in
extreme case where the datapath flow limit is exceeded.  1.5s was
chosen to ensure pings occurring at once per second stay cached in the
datapath.

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