ofproto: Increase default flow-eviction-threshold.

The flow-eviction-threshold presents a trade off between the
expense of maintaining large numbers of datapath flows, and the
benefit of avoid unnecessary flow misses.  In some large Open
vSwitch deployments, we've seen the previous default flow eviction
threshold negatively impact performance with reasonably typical
traffic patterns.  This patch increases the default to a level
which should represent a better trade off: still relatively safe,
but much more amenable to large numbers of long lived flows.

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