Replace OUR_MAX_HTLCS constant with a config knob

[TheBlueMatt]

Basically what it says on the tin - add a new config knob and use it in place of OUR_MAX_HTLCS, writing an even TLV if its not 50 to break forwards compat.

[ariard]

The config knob should warn the post-anchor fee-bumping reserves should be scale in consequence (or we do that under the hood). And probably few other warnings.

[swilliamson5]

I'd like to help! I'll take this as a first issue. I'll need some guidance and I might also post in the Discord for help if needed.

[swilliamson5]

Sorry for the delay, working on trying to figure out how to properly test this. I'm relatively new to LDK and Lightning development so I could be in over my head here. Here's a code snippet for the "even TLV" change. Also, I added a new field to the Channel struct in channel.rs to replace the referenced constant where possible. Let me know if I am way off!

write_tlv_fields!(writer, {
...
  (27, self.channel_keys_id, required),
  (28, Some(self.max_accepted_htlcs), option),
});

[valentinewallace]

Sorry for the delay, working on trying to figure out how to properly test this. I'm relatively new to LDK and Lightning development so I could be in over my head here. Here's a code snippet for the "even TLV" change. Also, I added a new field to the Channel struct in channel.rs to replace the referenced constant where possible. Let me know if I am way off!

write_tlv_fields!(writer, {
...
  (27, self.channel_keys_id, required),
  (28, Some(self.max_accepted_htlcs), option),
});

You can write it as required and read it as an option, which is a bit more succinct, but what you have would work. Matt also mentioned in the top-level comment that it should only be written if it's not 50.

But, this sounds like a fine start, feel free to open a draft PR and you can get feedback there :)