[tests/querier] use the new connection api (part 3/3)
Some adaptations to devtools/repotest:
* dead code removal
* remove session related code
In the tests:
* many tests actually don't check the querier but some generic rql
property -- when such a test needs several statements to complete a
commit, switch to session.new_cnx
* provide an assertRQLEqual helper
drop_relation_definition('CWSourceSchemaConfig', 'cw_schema', 'CWAttribute')
sync_schema_props_perms('cw_schema')