classifier: Support duplicate rules.

classifier: Support duplicate rules.

OpenFlow 1.4 bundles are easier to implement when it is possible to
mark a rule as 'to_be_removed' and then insert a new, identical rule
with the same priority.

All but one out of the identical rules must be marked as
'to_be_removed', and the one rule that is not 'to_be_removed' must
have been inserted last.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>