Java API: split incRef into incRef and addToReferenceQueue

One method should do one thing only, it's easy to mix things up
otherwise.

src/api/java/Constructor.java

@@ -85,9 +85,12 @@ public class Constructor extends Z3Object {
     }
 
     @Override
-    void incRef(long o) {
-
+    void incRef() {
         // Datatype constructors are not reference counted.
+    }
+
+    @Override
+    void addToReferenceQueue() {
         getContext().getConstructorDRQ().storeReference(getContext(), this);
     }