JPA DAOs must ensure that property keys are unique

Currently, writeVariableProperty creates always new entries to the XmlDataProperty table, even if the key is identical.This break correlation in ODE 2.0 as properties are then stored even in the case of variable initialization. This typically results in an empty property value, which will not be replaced by upcoming property write operations (they will simply add new objects with the same key to the table, property reads will always return the first occurrence of the key, which represents in that case an empty value).

Although this is not directly affecting 1.x, making sure that the key is unique doesn't harm 1.x either.