PPPoE usses a midchain adjacency stack on an interface-tx DPO

PPPoE usses a midchain adjacency stack on an interface-tx DPO

1) introduce an interface-tx DPO. This is a simple wrapper around a sw_if_index. enhance DPO stacking functions to allow per-instance next-nodes and hence allow children to stack onto the interface per-instance tx node and not on 'interface-output'.
2) update PPPoE code to use ta midchain stack on a interface-tx DPO of the encap-interface. This remove the need for pppoe_encap node (which is replaced by the adj-midchain-tx) and interface-output node is no longer used (see above). Since PPPoE encap node is no longer needed, the PPPoE seesion does not need to be retrieved in the data-path, hence the cahce misses are removed.

Change-Id: Id8b40f53daa14889a9c51d802e14fed7fba4399a
Signed-off-by: Neale Ranns <nranns@cisco.com>