node: keep track of roam-sources and roam-destinations

node: keep track of roam-sources and roam-destinations

Keep track of STA journeys between multiple nodes. When a new STA is
reported from a foreign node, check whether this STA was previously
associated with a local node.

Vice versa, check upon association whether or not the STA was previously
associated with a foreign node.

Keep track of these roaming transactions using two new (local) fields of
the node struct.

Signed-off-by: David Bauer <mail@david-bauer.net>