Add heuristic to LDK code or docs

[moneyball]

From LDK Discord, @TheBlueMatt:

"Because there’s rarely liquidity limits for reasonable-value payments in the “core” of ln, one trivial heuristic would be to decline to send a pre-flight probe if the amount is > 1/3 of your available outbound liquidity on a channel that’s used for the probe"

"presumably ldk-node should do that for you, but we may want to document these guidelines in rust-lightning too
or have a wrapper which does the checks pre-flight in ChannelManager"

[TheBlueMatt]

Should be a nice and easy project for someone - just a new ChannelManager method called pre-flight probing or whatever that takes some payment info and fetches a route, sending probes for any paths which use less than 1/3 of the available liquidity on the first-hop.

[ariard]

I think can be related to #2397

[TheBlueMatt]

Not really - this issue is in reference to Just In Time probing, and adding the (trivial) method to do that for a single payment. 2397 is about background probing and the machinery to pick who to probe, how often to probe, and do so constantly unrelated to payments.

[jbesraa]

keen to pick this up.
I have 2 open prs but one of them looks complete so I hope its ok to pick this.

[tnull]

keen to pick this up. I have 2 open prs but one of them looks complete so I hope its ok to pick this.

Ah, thank you! However, we concluded in lightningdevkit/ldk-node#156 that we'd upstream the code from LDK Node, so the actual work is essentially already done, will open an PR for it soon.

[jbesraa]

cool, will pick something else 👯

[TheBlueMatt]

I have 2 open prs but one of them looks complete so I hope its ok to pick this.

Probably a good indication you should go review some other PRs :p