Move lock up, to cover more variables.
Replace find method for [] operator on some maps.
Change some Readable by Openable, as that's what we're really testing.
Change units from MB to MiB, as that's what we are handling.
Remove upbound limit for prune configuration, and set default at maximum if configured below minimum.

Reasoning is that if prune is set, the user must want it. And if it's configured erroneously,
setting at maximum should be the safer mode of operation.
Pass a boolean instead of a string to RemoveDiskFile.
Remove checking size on disk.