MINOR: pools: split the OS-based allocator in two

MINOR: pools: split the OS-based allocator in two

Now there's one part dealing with the allocation itself and keeping
counters up to date, and another one on top of it to return such an
allocated pointer to the user and update the use count and stats.

This is in anticipation for being able to group cache-related parts.
The release code is still done at once.