airbase: cleanup recovered aircraft units

[jtoppins]

By default ED has a very long deletion timer for recovered aircraft. With the airbase asset we should be able to control better the deletion of these units by listening to the land event.

Initial types of recovery and possible behaviors can be found here: https://github.com/jtoppins/dct/blob/master/doc/03-designer.md#recoverytype

Issues:
The unit will need to be checked against the list of player groups and ignored if it belongs to a player group.