Allow outgoing splice request while disconnected

This is crucial for peers that serve liquidity for low-availability
(i.e., mobile) nodes. We should allow users to queue a splice request
while the peer is offline, such that it is negotiated once reconnected.
Note that there currently isn't a way to time out/cancel these requests,
this is planned for the near future.

Author: wpaulino

lightning/src/ln/channel.rs

@@ -11507,10 +11507,10 @@ where
 			});
 		}
 
-		if !self.context.is_live() {
+		if !self.context.is_usable() {
 			return Err(APIError::APIMisuseError {
 				err: format!(
-					"Channel {} cannot be spliced, as channel is not live",
+					"Channel {} cannot be spliced as it is either pending open/close",
 					self.context.channel_id()
 				),
 			});
@@ -12656,13 +12656,15 @@ where
 			|| self.context.channel_state.is_awaiting_quiescence()
 			|| self.context.channel_state.is_local_stfu_sent()
 		{
+			log_info!(logger, "Channel is either pending or already quiescent");
 			return Ok(None);
 		}
 
 		self.context.channel_state.set_awaiting_quiescence();
 		if self.context.is_live() {
 			Ok(Some(self.send_stfu(logger)?))
 		} else {
+			log_info!(logger, "Waiting for peer reconnection to send stfu");
 			Ok(None)
 		}
 	}

lightning/src/ln/channelmanager.rs

@@ -4675,14 +4675,6 @@ where
 		// Look for the channel
 		match peer_state.channel_by_id.entry(*channel_id) {
 			hash_map::Entry::Occupied(mut chan_phase_entry) => {
-				if !chan_phase_entry.get().context().is_connected() {
-					// TODO: We should probably support this, but right now `splice_channel` refuses when
-					// the peer is disconnected, so we just check it here.
-					return Err(APIError::ChannelUnavailable {
-						err: "Cannot initiate splice while peer is disconnected".to_owned(),
-					});
-				}
-
 				let locktime = locktime.unwrap_or_else(|| self.current_best_block().height);
 				if let Some(chan) = chan_phase_entry.get_mut().as_funded_mut() {
 					let logger = WithChannelContext::from(&self.logger, &chan.context, None);