ofproto-dpif.at: Fix a race in "ofproto-dpif - controller"

Simon Horman submitted the identical patch independently.

Acked-by: Ben Pfaff <blp@nicira.com>
Tested-by: Lori Jakab <lojakab@cisco.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
ofproto-dpif.at: Avoid races in table miss tests

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
learn.at: Fix a race in "self-modifying flow with idle_timeout" test

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
ofproto-dpif.at: Fix some races in MPLS actions tests

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
ofproto-dpif.at: Fix a race in fin_timeout test

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
ofproto-dpif.at: Fix a race in "flow stats" test

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
ofproto-dpif.at: Fix races in "NetFlow flow expiration" tests

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
learn.at: Fix a race in "self-modifying flow with hard_timeout"

Also, add a comment to explain the expected sequences.

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
learn.at: Fix a race in "learn refreshes hard_age" test

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
ofproto-dpif.at: Fix a race in "idle_age and hard_age increase over time"

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>