Improve configuration for disk usage calculation

[mvz]

We should use actual bytes to configure the allocation unit, and allow autodetection.

Summary

As explained in the comments for #minimum_disk_space_used, the configuration of the disk space calculator uses 8-byte units. This is very confusing. We should also follow-up on the other TODOs in that comment.

Expected Behavior

Allocation unit configuration can be set to actual block size in bytes, or nil for autodetection.

Current Behavior

Size needs to be set in units of 8 bytes.

Context & Motivation

This will make the disk usage feature easier to use, especially if autodetection can be achieved.

[stale]

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