ofproto-dpif: Allow setting of flow eviction threshold

ofproto-dpif: Allow setting of flow eviction threshold

Allow setting the number of flows present in the flow hash
at which point eviction of entries from the kernel flow hash
will begin to occur.

The value may be set using a bridge's other-config column.

e.g.

ovs-vsctl set bridge br3 other-config:flow-eviction-threshold=10000

default is 1000, reflecting constant value previously used.

Increasing this value can result in reduced CPU usage and
packet loss in situations where the number of active flows
is significantly larger than 1000.