[CWEP002] properly handle serialization of computed attributes
Until this cset, only serialization of computed attribute's formula was
handled (thanks to the relation definition's properties dict).
We now:
* properly serialize/deserialize attribute's formula
* test first if the database has been migrated and contains related
column or not
Related to #3546717
[CWEP002] Add support for computed attribute synchronization
Related to #3546717.
Test and handle the behaviour with several formulas and identified use cases. To
do so add a birth year and a computed attribute age to Person in the test
schema.