mem: retrieve mempool cache only when needed
mem: retrieve mempool cache only when needed

It is an optimization for the single consumer case,
or when cache is too small,
or when cache is disabled.

Signed-off-by: Intel

mixja/dpdk,tsphillips/dpdk-fork,msune/dpdk,venkynv/dpdk-mirror,mixja/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,msune/dpdk,msune/dpdk,tsphillips/dpdk-fork,mixja/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,msune/dpdk,tsphillips/dpdk-fork,mixja/dpdk,venkynv/dpdk-mirror