fix referential, first implementation property
Fix plug and unplug in cache layer
Merge remote-tracking branch 'origin/dev' into erik

Conflicts:
	gs-kernel/src/test/java/org/genericsystem/kernel/AdjustMetaTest.java
	gs-kernel/src/test/java/org/genericsystem/kernel/PropertyConstraintTest.java
	gs-kernel/src/test/java/org/genericsystem/kernel/RemovableServiceTest.java
	gs-kernel/src/test/java/org/genericsystem/kernel/WeakEquivTest.java
Problem when 2 holders add on concrete