JBPM-3916 - Keeping multi instance node (instance) ids consistent

This solves [JBPM-3916](https://issues.jboss.org/browse/JBPM-3916) as well as some related issues. 

However, with regards to the ids for node instances, it obviously breaks backwards compatibility.. I'm not sure whether or not it's a good idea to merge it for 6.1, thus.. :/