async signing completion

[TheBlueMatt]

A few nits from #3109 (review) and then all we need is to handle async per-commitment-point in reconnection.

[TheBlueMatt]

cc @alecchendev cause I can't assign you.

[alecchendev]

thanks!

[moneyball]

Hi @alecchendev just wondering if this is something you plan to tackle?

[alecchendev]

Hey, sorry for the long delay, unfortunately I will not be picking this up anytime soon.