[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]
if versions_map['cubicweb'][0] == (3, 7, 4):
config['http-session-time'] *= 60
config['cleanup-session-time'] *= 60
config['cleanup-anonymous-session-time'] *= 60