cores
* Only keep a single core online most of the time.
* Device: Add an enableCPUCores method to allow controlling the amount of
online CPU cores.
* Move the initial core onlining setup to Kobo:init, instead of the startup script.
* Enable two CPU cores while hinting new (e.g., cache miss) pages in PDF land.
* Enable two CPU cores while processing book metadata.
* Drive-by fix to isolate the DocCache pressure check to KoptInterface
and actually apply it when it matters most (e.g., k2pdfopt stuff).
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