Parsed test spec with 3 sessions

starting permutation: w1_insert w2_insert w1_commit w2_commit check_count
step w1_insert: 
    INSERT INTO ulak.queue (endpoint_id, payload, idempotency_key)
    SELECT e.id, jsonb_build_object('from', 'w1'), 'idem-test-1'
    FROM ulak.endpoints e WHERE e.name = 'iso_idem_ep'
    RETURNING payload::text, idempotency_key;

payload       |idempotency_key
--------------+---------------
{"from": "w1"}|idem-test-1    
(1 row)

step w2_insert: 
    INSERT INTO ulak.queue (endpoint_id, payload, idempotency_key)
    SELECT e.id, jsonb_build_object('from', 'w2'), 'idem-test-1'
    FROM ulak.endpoints e WHERE e.name = 'iso_idem_ep'
    RETURNING payload::text, idempotency_key;
 <waiting ...>
step w1_commit: COMMIT;
step w2_insert: <... completed>
ERROR:  duplicate key value violates unique constraint "idx_queue_idempotency_key"
step w2_commit: COMMIT;
step check_count: 
    SELECT count(*) AS msg_count, min(payload::text) AS payload
    FROM ulak.queue
    WHERE idempotency_key = 'idem-test-1';

msg_count|payload       
---------+--------------
        1|{"from": "w1"}
(1 row)