Implement reference handling support

We need to implement the mechanism for handling (adding/removing) node references. A few implementation points:

• atm there are 2 methods: addReferrer and removeRefferrer on the MutableCachedNode interface, but they aren't used
• the original idea was to update the AbstractJcrNode #set/remove property methods and manage references from there, but this is not viable in the context of transactions and multiple threads
• given the above, the correct place to implement this, probably is WritableSessionCache#persistChanges, since the DocumentTranslator does have enough information about the properties, to be able to tell whether they are reference properties or not (the runtime value of the properties should be a Reference instance)
• also, to handle the case when multiple properties reference the same node, we probably need a referenceCount field which would be used to check whether a reference actually needs removing or not