Handle htlc_max != channel capacity better in scoring

[TheBlueMatt]

There's two issues today with the way we handle channels with an htlc-max being different from the channel's capacity (which historically was never a thing but we expect to happen more over time, so should support it now):

• some users expect to operate without validating on chain UTXOs. We should (a) ensure rapid gossip sync includes actual channel capacity and (b) update docs to make clearer that without validation of UTXOs we will score incorrectly,
• we may think the channel balance available is incorrect as we calculate based on the wrong value. This may mostly already be right but one concern is how do we differentiate between a channels max-in-flight being maxed out vs the channel's balance being actually maxed out. It's unclear to me that that's possible or how we'd account for it.

[jkczyz]

• we may think the channel balance available is incorrect as we calculate based on the wrong value. This may mostly already be right but one concern is how do we differentiate between a channels max-in-flight being maxed out vs the channel's balance being actually maxed out. It's unclear to me that that's possible or how we'd account for it.

To add more context and clarification, the EffectiveCapacity of the channel is used during scoring. Currently, the channel capacity is preferred over htlc_maximum_msat for the EffectiveCapacity when both are know. If the capacity is unknown, htlc_maximum_msat is used.

[tnull]

I plan to pick this up eventually.