Support 0conf payments

[TheBlueMatt]

This has been in the works for some time, with several PRs already landed, but I don't see an issue for it, so we should have one.

[ViktorTigerstrom]

@TheBlueMatt I know you're already working on implementing support for this, but is there any subpart of this issue where it would be helpful to make some contribution to? Or would it be more helpful to find another issue to work on instead?

[TheBlueMatt]

Hmm, I don't think so sadly. Most of the work is done, its just a matter of writing tests and updating as the spec changes. If you're looking for things to work on #1279 and #1280 are probably good candidates.

[ViktorTigerstrom]

Great thanks, I'll look into those instead.

[TheBlueMatt]

Merged!