ofproto/bond: Implement bond megaflow using recirculation

Infrastructure to enable megaflow support for bond ports using
recirculation. This patch adds the following features:
* Generate RECIRC action when bond can benefit from recirculation.
* Populate post recirculation rules in a hidden table. Currently table 254.
* Uses post recirculation rules for bond rebalancing
* A recirculation implementation in dpif-netdev.

The goal of this patch is to be able to megaflow bond outputs and
thus greatly improve performance. However, this patch does not
actually improve the megaflow generation. It is left for a later commit.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>