Fix memory leak

You can check with yourkit but I think that the memory leak observed was linked to the image used as keypart

Please note that if you use a custom object as keypart you can use annotation to be able to generate a really unique key

see 2 examples here
https://github.com/JRebirth/JRebirth/blob/eae7964add62bfbd438d50b2a1fdd03e3174d157/org.jrebirth.af/core/src/test/java/org/jrebirth/af/core/ui/model/ModelBean.java

https://github.com/JRebirth/JRebirth/blob/eae7964add62bfbd438d50b2a1fdd03e3174d157/org.jrebirth.af/core/src/test/java/org/jrebirth/af/core/ui/model/ModelBean2.java