lnwire: refactor Encode to use specific writers - III

This commit refactors the remaining usage of WriteElements. By
replacing the interface types with concrete types for the params used in
the methods, most of the encoding of the messages now takes zero heap
allocations.

lnwire/lnwire_test.go

@@ -232,7 +232,7 @@ func TestMaxOutPointIndex(t *testing.T) {
 	}
 
 	var b bytes.Buffer
-	if err := WriteElement(&b, op); err == nil {
+	if err := WriteOutPoint(&b, op); err == nil {
 		t.Fatalf("write of outPoint should fail, index exceeds 16-bits")
 	}
 }