== Pulsar: the subscription type decides ==

12 messages, keys A/B/C, values 1..12 in order, two consumers per
subscription, receiverQueueSize 1 so the first consumer cannot take the
whole backlog.

Shared subscription, which consumers saw each key:
  A -> [consumer-1, consumer-2]
  B -> [consumer-1, consumer-2]
  C -> [consumer-1, consumer-2]

Key_Shared subscription, which consumers saw each key:
  A -> [consumer-1]
  B -> [consumer-1]
  C -> [consumer-1]

A Shared subscription round-robins individual messages, so messages with
the same key end up on different consumers and can be processed at the
same time: there is no per-key order left to speak of. Key_Shared hashes
the key to one consumer, which is Kafka's guarantee -- except that the
assignment belongs to the subscription and is recomputed as consumers come
and go, rather than being fixed by a partition count chosen when the topic
was created.