== Pulsar: acknowledged, but still there ==

subscription replay-sub, first read        : 12 messages
subscription replay-sub, after seek(earliest): 12 messages
subscription replay-sub-2, brand new       : 12 messages

Acknowledgement moves a cursor; the message itself lives in the managed
ledger. seek(MessageId) and seek(timestamp) rewind a live subscription,
which Kafka can also do by resetting offsets. What differs is the default:
Pulsar deletes a message once every subscription has acknowledged it,
unless a retention policy on the namespace says otherwise, whereas Kafka
keeps it for the retention period regardless of who read it. A Pulsar
topic with no retention policy and no subscriptions keeps nothing.