batman-adv: roaming handling mechanism redesign

This patch allows clients to roam multiple times within the same
originator-interval.

To enable this new feature two key aspects that have been introduced:
1) packets are always directed to the node that was originally
serving the roamed client which will then re-route the data
to the correct destination at any point in time;
2) the client flags handling mechanism has been properly modified
in order to allow multiple roamings withinin the same orig-int.
Therefore flags are now set properly even in this scenario.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>