OFStateManager: remove use of flow_add from flow stats handler

Instead use the flowtable entry fields to populate the of_flow_stats_entry.

I also noticed the flags field is missing from the LOXI of_flow_stats_entry
class (replaced by padding). I'll fix that separately.
OFStateManager: remove use of flow_add from flow removal

Instead use the flowtable entry fields to populate the of_flow_removed.

Also populated the table_id and hard_timeout fields which were added in later
OpenFlow versions.
OFStateManager: remove flow_add from flowtable entry

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