bonding: Add /if/lacp/<bond-sw_if_index>/<slave-sw_if_index>/partner-state

bonding: Add /if/lacp/<bond-sw_if_index>/<slave-sw_if_index>/partner-state

We already had /if/lacp/<bond-sw_if_index>/<slave-sw_if_index>/state in
the stats segment. Add also the partner-state to be complete.
Change to populate stats segment with the states at startup, after processing
an lacp pdu, and after timer expiration.

Unit test
---------
sudo vpp_get_stats dump | grep /if/lacp
63.00 /if/lacp/3/1/state
61.00 /if/lacp/3/1/partner-state
63.00 /if/lacp/3/2/state
61.00 /if/lacp/3/2/partner-state

Type: feature

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: Ib7b8e1183d572bb6e422a846aaa2b7b3559a0dc7