ofproto-dpif: Set flow-eviction-threshold globally.

ofproto-dpif: Set flow-eviction-threshold globally.

With the single datapath, it no longer makes sense to have a per
ofproto flow eviction threshold.  This patch moves the flow
eviction threshold to the Open_vSwitch table making the setting
global, though still treated separately for each ofproto.  A future
patch will unify flow eviction on a per datapath basis.

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