routing+routerrpc: improve prob. estimation for untried connections

This commit changes mission control to partially base the estimated
probability for untried connections on historical results obtained in
previous payment attempts. This incentivizes routing nodes to keep all
of their channels in good shape.

routing/missioncontrol_test.go

@@ -34,7 +34,8 @@ var (
 	mcTestNode2 = mcTestRoute.Hops[1].PubKeyBytes
 
 	testPenaltyHalfLife       = 30 * time.Minute
-	testAprioriHopProbability = 0.8
+	testAprioriHopProbability = 0.9
+	testAprioriWeight         = 0.5
 )
 
 type mcTestContext struct {
@@ -78,6 +79,7 @@ func (ctx *mcTestContext) restartMc() {
 		&MissionControlConfig{
 			PenaltyHalfLife:       testPenaltyHalfLife,
 			AprioriHopProbability: testAprioriHopProbability,
+			AprioriWeight:         testAprioriWeight,
 		},
 	)
 	if err != nil {
@@ -136,20 +138,23 @@ func TestMissionControl(t *testing.T) {
 
 	testTime := time.Date(2018, time.January, 9, 14, 00, 00, 0, time.UTC)
 
-	// Initial probability is expected to be 1.
-	ctx.expectP(1000, 0.8)
+	// Initial probability is expected to be the a priori.
+	ctx.expectP(1000, testAprioriHopProbability)
 
 	// Expect probability to be zero after reporting the edge as failed.
 	ctx.reportFailure(1000, lnwire.NewTemporaryChannelFailure(nil))
 	ctx.expectP(1000, 0)
 
 	// As we reported with a min penalization amt, a lower amt than reported
-	// should be unaffected.
+	// should return the node probability, which is the a priori
+	// probability.
 	ctx.expectP(500, testAprioriHopProbability)
 
-	// Edge decay started.
+	// Edge decay started. The node probability weighted average should now
+	// have shifted from 1:1 to 1:0.5 -> 60%. The connection probability is
+	// half way through the recovery, so we expect 30% here.
 	ctx.now = testTime.Add(30 * time.Minute)
-	ctx.expectP(1000, 0.4)
+	ctx.expectP(1000, 0.3)
 
 	// Edge fails again, this time without a min penalization amt. The edge
 	// should be penalized regardless of amount.
@@ -159,11 +164,11 @@ func TestMissionControl(t *testing.T) {
 
 	// Edge decay started.
 	ctx.now = testTime.Add(60 * time.Minute)
-	ctx.expectP(1000, 0.4)
+	ctx.expectP(1000, 0.3)
 
 	// Restart mission control to test persistence.
 	ctx.restartMc()
-	ctx.expectP(1000, 0.4)
+	ctx.expectP(1000, 0.3)
 
 	// A node level failure should bring probability of all known channels
 	// back to zero.