OFStateManager: remove use of flow_add from flow stats handler

Instead use the flowtable entry fields to populate the of_flow_stats_entry.
OFStateManager: remove use of flow_add from flow removal

Instead use the flowtable entry fields to populate the of_flow_removed.
OFStateManager: remove flow_add from flowtable entry

This saves about 18% RSS for an IVS with a full flowtable.