[schema2sql] properly consider skip_relations parameters.
This is the same bug as yams #286912. Also, introducing a unique function
telling if the relation should have a table or not unhide some other problems
related to computed relation, or missing skipped relations information.
change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')