vswitchd: Reduce flow idle time when flow table grows large.

vswitchd: Reduce flow idle time when flow table grows large.

This change halves the number of steady-state flows in the flow table
for hping3 --faster --quiet, from over 5000 to less than 2500.  It does
cause some oscillation in flow table size, because there is a harsh step
function in idle time when the flow table goes from 1000 to 1001 flows,
from 2001 to 2002 flows, and from 4003 to 4004 flows, but I doubt that is
a problem.  (If it is, we can introduce some randomness.)