Don't indirect map values

We already have reference behaviour, so this is not necessary.

This change highlighted fairly baroque architecture in mql that deserves
some attention; the use of channels is somewhat confusing.

Also rename LastResult to Result.

graph/iterator/and_iterator_test.go

@@ -44,7 +44,7 @@ func TestTag(t *testing.T) {
 		t.Errorf("Unexpected value")
 	}
 	tags := make(map[string]graph.TSVal)
-	and.TagResults(&tags)
+	and.TagResults(tags)
 	if tags["bar"] != 234 {
 		t.Errorf("no bar tag")
 	}