batman-adv: Don't keep redundant TT change events

When adding a local TT twice within the same OGM interval (e.g. happens
when flag get updated), the flags of the first TT change entry is updated
with the second one and both change events is added to the change list.
This leads to having the same ADD change entry twice. Similarly, a
DEL+DEL scenario is also creating twice the same event.

Deduplicate ADD+ADD or DEL+DEL scenarios to reduce the TT change events
that need to be sent in both OGM and TT response.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Co-developed-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>