Planner: Immovable planning entities for chained variables

Immovable planning entities are critical for continuous planning.
However, the combination of immovable planning entities and chained variables has been broken for year and it's one of OptaPlanner's hotests bugs in the community jira:
https://issues.jboss.org/browse/PLANNER-239

I've found a way to fix it for 7.0 and I can backport it to 6.4.x (= BRMS 6.3) if this is triaged.

This issue is critical for task assigning that uses the chained through time pattern (2 customers do this).