Optim denunciation pool cache cleanup

[sydhds]

• document all added functions
• try in sandbox /simulation/labnet
• unit tests on the added/changed features
  • make tests compile
  • make tests pass
• add logs allowing easy debugging in case the changes caused problems
• if the API has changed, update the API specification

[damip]

how about directly doing:

let earliest_allowed_period = slot_period.saturating_sub(denunciation_expire_periods.saturating_sub(1));
*cache = cache.split_off(
    &DenunciationIndex::Endorsmeent{ slot: Slot::new(earliest_allowed_period, 0), index: 0 }
);

?

[damip]

Good work !