[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]
sync_schema_props_perms('cw_source', syncprops=False)
if schema['BigInt'].eid is None:
add_entity_type('BigInt')