Rate upgrade: disconnect peer upon session switch from slave interface

Following successful session switch, when the old interface is not the
group's master, send disconnect command on old interface.

On AP side link loss detection takes long time and can cause data traffic
stale because network stack is unaware of link/iface state change.
To workaround this, FST Manager explicitly requests disconnect when the
data traffic is switched away from the slave interface.

Change-Id: I8331178c3a90e1fcbbcda19b1fbaab319945f729