flow-stats-reply handling rework

Reviewer: @rlane

To detect connection_t staleness, add generation_id and utility routines to convert a connection_t* to a void* cookie.

Use this void* cookie for OF object tracking, to prevent crashes when an OF object associated with a stale connection is deleted.

Convert flow_stats handler to a long running task.

Increase WRITE_BUFFER_SIZE to handle a 129k flow stats reply.