Decay `historical_estimated_channel_liquidity_*` result to `None`

Decay `historical_estimated_channel_liquidity_*` result to `None`

`historical_estimated_channel_liquidity_probabilities` previously
decayed to `Some(([0; 8], [0; 8]))`. This was thought to be useful
in that it allowed identification of cases where data was previously
available but is now decayed away vs cases where data was never
available. However, with the introduction of
`historical_estimated_payment_success_probability` (which uses the
existing scoring routines so will decay to `None`) this is
unnecessarily confusing.

Given data which has decayed to zero will also not be used anyway,
there's little reason to keep the old behavior, and we now decay to
`None`.

We also take this opportunity to split the overloaded
`get_decayed_buckets`, removing uneccessary code during scoring.