Efficient utxo management

[JssDWt]

Currently lspd doesn't do its own utxo management for utxos used in channel opens. The lightning node itself is in charge of reserving utxos and crafting transactions that will be used for channel funding. Lightning nodes, however, currently don't have any smart heuristics for which utxo to choose for a channel open. They will either pick the largest utxo or a random one. Lspd opens channels with very specific patterns though, so utxo usage can be smarter.

Some thoughts:

• chainfees for zero conf channel opens could generally be on the low end, as long as eventually a child pays for the confirmation of its parent. This mechanism can be used to bridge temporary high fee environments.
• In some occasions it can be smart to open a slightly larger channel if there's a utxo that can be used without creating a change output
• In general, however, it could be a good idea to always have a change output attached to a channel open tx, so that can be used to bump the tx fee using CPFP to ensure a timely open of the channel (node implementations forget channels by default after 2016 blocks)
• Channel open transactions can be used to consolidate smaller utxos.

This is not a small task, and there are many things that will have to be taken into consideration. Especially the fact that there should never be a possibility for a double spend of a funding transaction.