Update last_datapoint_time even if we don't update bounds

When we see a failure or success and it doesn't result in a bounds
update, we previously were'nt updating the `last_datapoint_time`
field as we were updating it in the bounds-update logic.

This is wrong for the purpose of the `probing_diversity_penalty`
because the whole point is to avoid repeatedly probing the same
channel, but because of this we'll probe the same channel again and
again as long as we don't learn any new information from the probes
which causes a bounds update.

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -1556,6 +1556,7 @@ DirectedChannelLiquidity<L, HT, T> {
 				chan_descr, existing_max_msat, amount_msat);
 		}
 		self.update_history_buckets(0, duration_since_epoch);
+		*self.last_datapoint_time = duration_since_epoch;
 	}
 
 	/// Adjusts the channel liquidity balance bounds when failing to route `amount_msat` downstream.
@@ -1571,6 +1572,7 @@ DirectedChannelLiquidity<L, HT, T> {
 				chan_descr, existing_min_msat, amount_msat);
 		}
 		self.update_history_buckets(0, duration_since_epoch);
+		*self.last_datapoint_time = duration_since_epoch;
 	}
 
 	/// Adjusts the channel liquidity balance bounds when successfully routing `amount_msat`.
@@ -1580,6 +1582,7 @@ DirectedChannelLiquidity<L, HT, T> {
 		let max_liquidity_msat = self.max_liquidity_msat().checked_sub(amount_msat).unwrap_or(0);
 		log_debug!(logger, "Subtracting {} from max liquidity of {} (setting it to {})", amount_msat, chan_descr, max_liquidity_msat);
 		self.set_max_liquidity_msat(max_liquidity_msat, duration_since_epoch);
+		*self.last_datapoint_time = duration_since_epoch;
 		self.update_history_buckets(amount_msat, duration_since_epoch);
 	}
 
@@ -1603,7 +1606,6 @@ DirectedChannelLiquidity<L, HT, T> {
 			*self.max_liquidity_offset_msat = 0;
 		}
 		*self.last_updated = duration_since_epoch;
-		*self.last_datapoint_time = duration_since_epoch;
 	}
 
 	/// Adjusts the upper bound of the channel liquidity balance in this direction.
@@ -1613,7 +1615,6 @@ DirectedChannelLiquidity<L, HT, T> {
 			*self.min_liquidity_offset_msat = 0;
 		}
 		*self.last_updated = duration_since_epoch;
-		*self.last_datapoint_time = duration_since_epoch;
 	}
 }
 
@@ -4143,7 +4144,7 @@ mod tests {
 		scorer.payment_path_failed(&payment_path_for_amount(500), 42, Duration::ZERO);
 
 		// Apply an update to set the last-update time to 1 second
-		scorer.payment_path_failed(&payment_path_for_amount(490), 42, Duration::from_secs(1));
+		scorer.payment_path_failed(&payment_path_for_amount(500), 42, Duration::from_secs(1));
 
 		// If no time has passed, we get the full probing_diversity_penalty_msat
 		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 1_000_000);