Think of whether queue folder names / filenames should be configurable

[Ekleog]

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Currently they're const-set to "data", "queue", "inflight" and "cleanup", and the files in a mail folder are "contents", "metadata" and "schedule".

And we hardcode new queue id generation to be a new UUID.

Such a change is probably useless, but… should we make all these parameters configurable? Out of the blue I'd guess no, but would be happy to hear contradictory arguments.