[schema] define full_rql on RQLExpression class without it, we may have an error while displaying error which occured during some expression initialization because this attribute is not yet defined while used in the __str__ method.

[schema] properly raise BadSchemaDefinition when some expression mains variables may not be guessed

[CWEP002] properly handle serialization of computed relations We now: * have CWComputedRelation in the bootstrap schema to store computed relations * properly serialize/deserialize it * test first if the database has been migrated and contains the related table Related to #3546717 [jcr: adjust unittest_querier to pass with the added entity type]