Fix: controller: ensure lost node's transient attributes are cleared without DC

Previously, peer_update_callback() cleared a lost node's transient attributes
if either the local node is DC, or there is no DC.

However, that left the possibility of the DC being lost at the same time as
another node -- the local node would still have fsa_our_dc set while processing
the leave notifications, so no node would clear the attributes for the non-DC
node.

Now, the controller has its own CPG configuration change callback, which sets a
global boolean before calling the usual one, so that peer_update_callback() can
know when the DC has been lost.