OPTIM: pool: improve needed_avg cache line access pattern

OPTIM: pool: improve needed_avg cache line access pattern

On an AMD EPYC 3rd gen, 20% of the CPU is spent calculating the amount
of pools needed when using QUIC, because pool allocations/releases are
quite frequent and the inter-CCX communication is super slow. Still,
there's a way to save between 0.5 and 1% CPU by using fetch-add and
sub-fetch that are converted to XADD so that the result is directly
fed into the swrate_add argument without having to re-read the memory
area. That's what this patch does.