routerrpc+zpay32: EstimateRouteFee overhaul

In this commit the mission control based fee estimation
is supplemented with a payment probe estimation which can
lead to more accurate estimation results. The probing
utilizes a hop-hint heurisic to detect routes through LSPs
in order to manually estimate fees to destinations behind
an LSP that would otherwise block the payment probe.

lnrpc/routerrpc/router_backend.go

@@ -1175,8 +1175,18 @@ func unmarshallHopHint(rpcHint *lnrpc.HopHint) (zpay32.HopHint, error) {
 	}, nil
 }
 
+// MarshalFeatures converts a feature vector into a list of uint32's.
+func MarshalFeatures(feats *lnwire.FeatureVector) []lnrpc.FeatureBit {
+	var featureBits []lnrpc.FeatureBit
+	for feature := range feats.Features() {
+		featureBits = append(featureBits, lnrpc.FeatureBit(feature))
+	}
+
+	return featureBits
+}
+
 // UnmarshalFeatures converts a list of uint32's into a valid feature vector.
-// This method checks that feature bit pairs aren't assigned toegether, and
+// This method checks that feature bit pairs aren't assigned together, and
 // validates transitive dependencies.
 func UnmarshalFeatures(
 	rpcFeatures []lnrpc.FeatureBit) (*lnwire.FeatureVector, error) {