LN-DLC: Allow force-close at any point

[luckysori]

We must let consumers trigger force-close at any point in the protocol.

I think we will need to handle some states differently. For instance, with an Offered state I think we would need to fall back to the previous commitment transaction (sans split transaction), whereas with CloseConfirmed we would need to use the newly negotiated state (sans split transaction too).

I open this issue because of this check which ensures that the subchannel is in the Signed state:

rust-dlc/dlc-manager/src/sub_channel_manager.rs

Lines 709 to 714 in add0b0a

	let (mut signed, state) = get_sub_channel_in_state!(
	self.dlc_channel_manager,
	*channel_id,
	Signed,
	None::<PublicKey>
	)?;

[luckysori]

I think this whole issue has finally been addressed with #128.