Constraint Streams: support multiple groupBy calls

In some constraints, multiple groupBy calls might be necessary. See Machine reassignment : service move cost as an example:

return factory.from(MrProcessAssignment.class)
.filter(MrProcessAssignment::isMoved)
.groupBy(processAssignment -> processAssignment.getService(), ConstraintCollectors.countLong())
.groupBy((service, processMoveCount) -> processMoveCount)
.groupBy(ConstraintCollectors.max())
...