guarded-list: New data structure for thread-safe queue.

guarded-list: New data structure for thread-safe queue.

We already had queues that were suitable for replacement by this data
structure, and I intend to add another one later on.

flow_miss_batch_ofproto_destroyed() did not work well with the guarded-list
structure (it required either adding a lot more functions or breaking the
abstraction) so I changed the caller to just use udpif_revalidate().

Checking reval_seq at the end of handle_miss_upcalls() also didn't work
well with the abstraction, so I decided that since this was a corner case
anyway it would be acceptable to just drop those in flow_miss_batch_next().

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>