This commit adds per 'struct ofproto_dpif' sequence number for
packet-in I/O. Whenever ofproto_dpif_send_packet_in() is called,
the calling thread will change the sequence number to wake up the
main thread.
Signed-off-by: Alex Wang <alexw@nicira.com> Acked-by: Joe Stringer <joestringer@nicira.com>