Fix: initInstance hierarchy

* This problem was detected after adding DomainBPlusTree.  Problem: there
    was a bug in the invocation of LeafNode.setEntries() from the constructor
    of LeadNode. When actually creating an instance of DomainLeafNode, it
    would invoke the override DomainLeafNode.setEntries(), which would attempt
    to access a yet uninitialized slot (domainEntries).  The solution is to
    change how initInstance happens by invoking it only once from the
    top-level AbstractDomainObject class, instead of invoking it once per
    class hierarchy level.

@jcarvalho could you please review this PR and check that it doesn't break anything in jvstm-ojb? I only did a small run using FenixCodeGeneratorOneBoxPerObject and apparently it's ok (due to #141 I couldn't test with FenixCodeGenerator), but given that this commit affects the initInstance protocol, I'd like you to confirm it with a more serious test before merging.  In particular, I've disabled some of the processing done by PostProcessDomainClasses.AddOJBConstructorClassAdapter ([original here](https://github.com/fenix-framework/fenix-framework/blob/develop/core/dml-compiler/code-generator/src/main/java/pt/ist/fenixframework/core/PostProcessDomainClasses.java#L131)): I think that adding an invocation to initInstance in the 'allocation-only' constructor should no longer occur, because it's now being done in AbstractDomainObject's constructor.