Feature: Use a single scorer for each sim_node impl of Lightning Node

[carlaKC]

As is: The LightningNode impl for SimNode will create a new scorer every time it sends a payment.

To Be: scorer should be stored in SimNode and passed to pathfinding for re-use.

Motivation: our pathfinding heuristics start from fresh every time we send a payment, which makes pathfinding very dumb (it doesn't remember any of our previous attempts because we just throw them away). To better match reality we should track scores across payments.

While we're here:

• It could be nice to look into the payment parameters used by LDK and perhaps set something other than the default (if it makes sense). I've just never had the time but perhaps we can do something smarter here.