MINOR: pool: add a function to estimate how many may be released at once

MINOR: pool: add a function to estimate how many may be released at once

At the moment we count the number of releasable objects to a shared pool
one by one. The way the formula is made allows to pre-compute the number
of available slots, so let's add a function for that so that callers can
do it once before iterating.

This takes into account the average number of entries needed and the
minimum availability per pool. The function is not used yet.