See SQLBase._getExecutableMessageSet for operation principle.
Removes the notion of order_validation_text: activity validation is no
longer evaluated per-activity , but per-dependency for multiple activities
at a time. In this context, order_validation_text does not make sense as
it flattens all dependency types for a given activity.
Rework activity-dependency-to-SQL methods: use a dict rather
dynamically-generated method names.
Based on initial work by Julien Muchembled.