stats: add /if/<n>/<n>/state for lacp interface state

Add /if/lacp/<bond-sw_if_index>/<slave-sw_if_index>/state

<bond-sw_if_index> is a vector of the bond sw_if_index
<slave-sw_if_index> is a vector of the slave sw_if_index
Content is the integer value of the lacp actor state. The state is actually
a bitfield as described in the lacp protocol spec.

Type: feature

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: Ic6eca8ce2a1acd2d858e4e50b7eac1d000ea08e5
Signed-off-by: Ole Troan <ot@cisco.com>