revalidator: Replace ukey->mark with dump_seq.

Rather than setting and resetting the 'mark' field in the ukey, this
patch introduces a seq to track whether a flow has been seen during the
most recent dump. This tidies the code and simplifies the logic for
detecting when flows are duplicated from the datapath.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>