MEDIUM: pools: move the needed_avg counter over a few buckets

MEDIUM: pools: move the needed_avg counter over a few buckets

That's the same principle as for ->allocated and ->used. Here we return
the summ of the raw values, so the result still needs to be fed to
swrate_avg(). It also means that we now use the local ->used instead
of the global one for the calculations and do not need to call pool_used()
anymore on fast paths. The number of samples should likely be divided by
the number of buckets, but that's not done yet (better observe first).

A function pool_needed_avg() was added to report aggregated values for
the "show pools" command.

With this change, an h2load made of 5 * 160 conn * 40 streams on 80
threads raised from 1.5M RPS to 6.7M RPS.