ofproto-dpif: Use sequence number to wake up main thread for
packet-in I/O.

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>