MINOR: pools: get rid of POOL_EXTRA

MINOR: pools: get rid of POOL_EXTRA

This macro is build-time dependent and is almost unused, yet where it
cannot easily be avoided. Now that we store the distinction between
pool->size and pool->alloc_sz, we don't need to maintain it and we
can instead compute it on the fly when creating a pool. This is what
this patch does. The variables are for now pretty static, but this is
sufficient to kill the macro and will allow to set them more dynamically.