Refactor InvoicePayer to allow use of async event handler

Previously, an InvoicePayer's event_handler was only used in one place,
handle_event. We now keep the event interception intact, but create two
different wrapper structs that allow the InvoicePayer to implement both
sync and async event handlers.

Author: wpaulino

lightning-background-processor/src/lib.rs

@@ -630,7 +630,7 @@ mod tests {
 	use std::time::Duration;
 	use bitcoin::hashes::Hash;
 	use bitcoin::TxMerkleNode;
-	use lightning::routing::scoring::{FixedPenaltyScorer};
+	use lightning::routing::scoring::FixedPenaltyScorer;
 	use lightning_rapid_gossip_sync::RapidGossipSync;
 	use super::{BackgroundProcessor, GossipSync, FRESHNESS_TIMER};
 
@@ -1155,7 +1155,7 @@ mod tests {
 		let data_dir = nodes[0].persister.get_data_dir();
 		let persister = Arc::new(Persister::new(data_dir));
 		let router = DefaultRouter::new(Arc::clone(&nodes[0].network_graph), Arc::clone(&nodes[0].logger), random_seed_bytes, Arc::clone(&nodes[0].scorer));
-		let invoice_payer = Arc::new(InvoicePayer::new(Arc::clone(&nodes[0].node), router, Arc::clone(&nodes[0].logger), |_: _| {}, Retry::Attempts(2)));
+		let invoice_payer = Arc::new(InvoicePayer::new(Arc::clone(&nodes[0].node), router, Arc::clone(&nodes[0].logger), Retry::Attempts(2)).with_event_handler(|_: _| {}));
 		let event_handler = Arc::clone(&invoice_payer);
 		let bg_processor = BackgroundProcessor::start(persister, event_handler, nodes[0].chain_monitor.clone(), nodes[0].node.clone(), nodes[0].no_gossip_sync(), nodes[0].peer_manager.clone(), nodes[0].logger.clone(), Some(nodes[0].scorer.clone()));
 		assert!(bg_processor.stop().is_ok());