Change Gossip handler to set country_undesirability if exit-location is set

[czarte]

In the current state, the handling of the exit-location command seems to work fine on the Neighborhood database that exists at the moment it's invoked; but the regime it inaugurates must be imposed on all Nodes that are added to the database afterwards as well. That probably means you'll need to change the way Gossip modifies the database: anytime it puts a new Node in, that Node's country_undesirability to match the Neighborhood.

TODO

• introduce back the former structure of exit_location_opt: Option<Vec<ExitLocation>>
• change Gossip handler to invoke the exit-location handler part set_country_undesirability to set prefered countries undesirability for newly introduced nodes from gossip

[czarte]

this is going to be part of #468