routing: add inbound fee support to pathfinding

Add sender-side support for inbound fees in pathfinding
and route building.

routing/heap.go

@@ -18,10 +18,16 @@ type nodeWithDist struct {
 	// outgoing edges (channels) emanating from a node.
 	node route.Vertex
 
-	// amountToReceive is the amount that should be received by this node.
+	// netAmountReceived is the amount that should be received by this node.
 	// Either as final payment to the final node or as an intermediate
-	// amount that includes also the fees for subsequent hops.
-	amountToReceive lnwire.MilliSatoshi
+	// amount that includes also the fees for subsequent hops. This node's
+	// inbound fee is already subtracted from the htlc amount - if
+	// applicable.
+	netAmountReceived lnwire.MilliSatoshi
+
+	// outboundFee is the fee that this node charges on the outgoing
+	// channel.
+	outboundFee lnwire.MilliSatoshi
 
 	// incomingCltv is the expected absolute expiry height for the incoming
 	// htlc of this node. This value should already include the final cltv