Move Scorer requirement away from Router trait

We do this to enable users to create routers that do not need a scorer.
This can be useful if they are running a node the delegates pathfinding.

* Move `Score` type parameterization from `InvoicePayer` and `Router` to
`DefaultRouter`
* Adds a new field, `scorer`, to `DefaultRouter`
* Move `AccountsForInFlightHtlcs` to `DefaultRouter`, which we
will use to wrap the new `scorer` field, so scoring only happens in
`DefaultRouter` explicitly.
* Add scoring related functions to `Router` trait that we used to call
directly from `InvoicePayer`.
* Instead of parameterizing `scorer` in `find_route`, we replace it with
inflight_map so `InvoicePayer` can pass on information about inflight
HTLCs to the router.
* Introduced a new tuple struct, InFlightHtlcs, that wraps functionality
for querying used liquidity.

Author: jurvis

lightning-background-processor/src/lib.rs

@@ -1037,8 +1037,8 @@ mod tests {
 		// Initiate the background processors to watch each node.
 		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);
-		let invoice_payer = Arc::new(InvoicePayer::new(Arc::clone(&nodes[0].node), router, Arc::clone(&nodes[0].scorer), Arc::clone(&nodes[0].logger), |_: &_| {}, Retry::Attempts(2)));
+		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 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());