gossip_cyclon: refactor r6998, further tweak startup performance

Improve startup performance by manually sending two triggers on startup instead
of checking the state during initialisation.
gossip_cyclon: minor docu fixes
gossip_load: move value extraction to callback module
gossip: remove value extraction functions (get_values...)

Value extraction (get_values... messages) is very specific to the callback module
and should therefore be implemented in the api of the given callback module, not
the gossip module.
gossip_vivaldi: add skeleton implementing gossip_beh