fix!(chain): Make spk revelation return owned spks

[LLFourn]

Unfortunately there's no way to return a lifetime to something from a &mut self that doesn't extend the lifetime of the mutable borrow. So it's better not borrow at all here.

I think @evanlinjin tried to make this change some time ago but I NACK'd it. Now I understand why it's unavoidable. Sorry!

[notmandatory]

This needs a rebase but otherwise looks good to me.

[notmandatory]

@LLFourn is it OK if someone else takes this over and rebases it?

[ValuedMammal]

I think it can be done in #1438

[notmandatory]

Closing this PR as equivalent fixes are included in #1451. a5142decfcd0581bf871755b8130782666329017