MEDIUM: pools: spread the allocated counter over a few buckets

MEDIUM: pools: spread the allocated counter over a few buckets

The ->used counter is one of the most stressed, and it heavily
depends on the ->allocated one, so let's first move ->allocated
to a few buckets.

A function "pool_allocated()" was added to return the sum of the entries.
It's important not to abuse it as it does iterate, so everywhere it's
possible to avoid it by keeping a local counter, it's better. Currently
it's used for limited pools which need to make sure they do not allocate
too many objects. That's an acceptable tradeoff to save CPU on large
machines at the expense of spending a little bit more on small ones which
normally are not under load.