ofproto: Add selective Flow Removed messages and flow deletes (OpenFlow 0.9)

ofproto: Add selective Flow Removed messages and flow deletes (OpenFlow 0.9)

In OpenFlow 0.9, flow "expiration" messages are sent when flows are
explicitly removed by a delete action.  As such, the message is renamed
from Flow Expired to Flow Removed.  This commit adds that support as well
as supporting the ability to choose sending these messages on a per flow
basis.

NOTE: OVS at this point is not wire-compatible with OpenFlow 0.9 until the
final commit in this OpenFlow 0.9 set.