Probe only up to the last public hop if there's no other choices

[TheBlueMatt]

If we're doing pre-flight probing for a payment, and the last hop is from an invoice route hint, and there's only one option for the last-hop route, and the node is not public, there's no reason to probe through to the endpoint - we might as well probe to the second-to-last hop. Worse, probing to the endpoint may confuse some LSPs, causing them to open a JIT channel when there's no real payment yet. For ldk-node this should be ~easy, for LDK itself we probably want to do it as a part of #2499.

cc @tnull

[tnull]

Yup, will make sure to include it in #2499.