Change default "impossibility penalty" to one Bitcoin

In general we should avoid taking paths that we are confident will
not work as much possible, but we should be willing to try each
payment at least once, even if its over a channel that failed
recently. A full Bitcoin penalty for such a channel seems
reasonable - lightning fees are unlikely to ever reach that point
so such channels will be scored much worse than any other potential
path, while still being below `u64::max_value()`.

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -403,7 +403,7 @@ pub struct ProbabilisticScoringParameters {
 	/// If you wish to avoid creating paths with such channels entirely, setting this to a value of
 	/// `u64::max_value()` will guarantee that.
 	///
-	/// Default value: `u64::max_value()`
+	/// Default value: 1_0000_0000_000 msat (1 Bitcoin)
 	///
 	/// [`liquidity_penalty_multiplier_msat`]: Self::liquidity_penalty_multiplier_msat
 	/// [`amount_penalty_multiplier_msat`]: Self::amount_penalty_multiplier_msat
@@ -548,7 +548,7 @@ impl Default for ProbabilisticScoringParameters {
 			amount_penalty_multiplier_msat: 256,
 			banned_nodes: HashSet::new(),
 			anti_probing_penalty_msat: 250,
-			considered_impossible_penalty_msat: u64::max_value(),
+			considered_impossible_penalty_msat: 1_0000_0000_000,
 		}
 	}
 }
@@ -2165,7 +2165,10 @@ mod tests {
 	fn accounts_for_inflight_htlc_usage() {
 		let logger = TestLogger::new();
 		let network_graph = network_graph(&logger);
-		let params = ProbabilisticScoringParameters::default();
+		let params = ProbabilisticScoringParameters {
+			considered_impossible_penalty_msat: u64::max_value(),
+			..ProbabilisticScoringParameters::zero_penalty()
+		};
 		let scorer = ProbabilisticScorer::new(params, &network_graph, &logger);
 		let source = source_node_id();
 		let target = target_node_id();