[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

[test/querier] use the new connection api (part 2/3) A couple of tests need a schema constraint to be dropped to pass with `.qexecute`.