Add `probing_diversity_penalty_msat` to the scorer parameters

Add `probing_diversity_penalty_msat` to the scorer parameters

When doing background probing, its important to get a diverse view
of the network graph to ensure you have as many options when
pathfinding as possible.

Sadly, the naive probing we currently recommend users do does not
accomplish that - using the same success-optimized pathfinder when
sending as when probing results in always testing the same (good)
path over and over and over again.

Instead, here, we add a `probing_diversity_penalty_msat` parameter
to the scorer, allowing us to penalize channels for which we
already have recent data. It applies a penalty which scales with
the square of the inverse time since the channel was last updated,
up to one day.