Reduce the default default channel bounds half-life

Utilizing the results of probes sent once a minute to a random node
in the network for a random amount (within a reasonable range), we
were able to analyze the accuracy of our resulting success
probability estimation with various PDFs across the historical and
live-bounds models.

For each candidate PDF (as well as other parameters, to be tuned in
the coming commits), we used the `min_zero_implies_no_successes`
fudge factor in `success_probability` as well as a total
probability multiple fudge factor to get both the historical
success model and the a priori model to be neither too optimistic
nor too pessimistic (as measured by the relative log-loss between
succeeding and failing hops in our sample data).

Across the simulation runs, for a given PDF and other parameters,
we nearly always did better with a shorter half-life (even as short
as 1ms, i.e. only learning per-probe rather than across probes).
While this likely makes sense for nodes which do live probing, not
all nodes do, and thus we should avoid over-biasing on the dataset
we have.

While it may make sense to only learn per-payment and not across
payments, I can't fully rationalize this result and thus want to
avoid over-tuning, so here we reduce the half-life from 6 hours to
30 minutes.

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -770,7 +770,7 @@ pub struct ProbabilisticScoringDecayParameters {
 	/// liquidity bounds are 200,000 sats and 600,000 sats, after this amount of time the upper
 	/// and lower liquidity bounds will be decayed to 100,000 and 800,000 sats.
 	///
-	/// Default value: 6 hours
+	/// Default value: 30 minutes
 	///
 	/// # Note
 	///
@@ -782,7 +782,7 @@ pub struct ProbabilisticScoringDecayParameters {
 impl Default for ProbabilisticScoringDecayParameters {
 	fn default() -> Self {
 		Self {
-			liquidity_offset_half_life: Duration::from_secs(6 * 60 * 60),
+			liquidity_offset_half_life: Duration::from_secs(30 * 60),
 			historical_no_updates_half_life: Duration::from_secs(60 * 60 * 24 * 14),
 		}
 	}
@@ -792,7 +792,7 @@ impl Default for ProbabilisticScoringDecayParameters {
 impl ProbabilisticScoringDecayParameters {
 	fn zero_penalty() -> Self {
 		Self {
-			liquidity_offset_half_life: Duration::from_secs(6 * 60 * 60),
+			liquidity_offset_half_life: Duration::from_secs(30 * 60),
 			historical_no_updates_half_life: Duration::from_secs(60 * 60 * 24 * 14),
 		}
 	}