An inbox in memory, for tests and single-process hosts: entries are copied in and out.
Stores a new entry; false, and nothing changes, when an entry with its id was accepted before (acknowledged or not).
Drops the entry's payload once the receiver has acted on it; its id stays, so a replay is still refused.
The entries accepted and not yet acknowledged, oldest first.
An inbox in memory, for tests and single-process hosts: entries are copied in and out.