Attempt queued splice after existing pending splice becomes locked

Since we don't yet support contributing to an incoming splice, we need
to make sure we attempt our splice negotiation eventually if the
counterparty was also attempting a splice at the same time but they won
the quiescence tie-breaker. Since only one pending splice (without RBF)
is allowed at a time, we do this after the existing splice becomes
locked.

Author: wpaulino

lightning/src/ln/channel.rs

@@ -10931,6 +10931,12 @@ where
 		let announcement_sigs =
 			self.get_announcement_sigs(node_signer, chain_hash, user_config, block_height, logger);
 
+		if let Some(quiescent_action) = self.quiescent_action.as_ref() {
+			if matches!(quiescent_action, QuiescentAction::Splice(_)) {
+				self.context.channel_state.set_awaiting_quiescence();
+			}
+		}
+
 		Some(SpliceFundingPromotion {
 			funding_txo,
 			monitor_update,