This functions more like the serial number strategy of the ecache and
hpa_central_t. Longer-lived slabs are more likely to continue to live for
longer in the future.
This introduces a new sort of edata_t; a pageslab, and a set to manage them.
This is part of a series of a commits to implement a hugepage allocator; the
pageset will be per-arena, and track small page allocations requests within a
larger extent allocated from a centralized hugepage allocator.
