autopilot/interface+prefattach: scale node scores to range [0.0, 1.0]

To prepare for combinning scores from multiple heuristics, we require the
scores returned from the NodeSores API to be in the range [0.0, 1.0].

The prefAttach heuristic is altered to scale the returned scores such
that the most connected node in the grpah is given a score of 1.0.

autopilot/prefattach_test.go

@@ -249,8 +249,8 @@ func TestPrefAttachmentSelectTwoVertexes(t *testing.T) {
 
 				// Since each of the nodes has 1 channel, out
 				// of only one channel in the graph, we expect
-				// their score to be 0.5.
-				expScore := float64(0.5)
+				// their score to be 1.0.
+				expScore := float64(1.0)
 				if candidate.Score != expScore {
 					t1.Fatalf("expected candidate score "+
 						"to be %v, instead was %v",