Async + Fallible Signing

[TheBlueMatt]

#1538 attempted to make our signing fallible (eg "usb signer is disconnected"). Since we redid the monitor/commitment-signing pipelines this should be much more practical now. However, when we do this we should also consider having the pipelines all be optionally-async. This would allow for proper remote signing without everything being blocked on RPC calls which is the only way to do it today.