Fixed: the AvatarEnteringNewParcel event wasn't triggered in some cases

If an avatar moved between regions: A -> B -> A, then when returning to region A the AvatarEnteringNewParcel wasn't triggered. This happened because the ScenePresence in region A still remembered its previous 'currentParcelUUID', so it appeared as if the avatar didn't change parcels. Now, however, when a ScenePresence becomes a child presence we clear its 'currentParcelUUID'.