mempool: add stack mempool handler

mempool: add stack mempool handler

This is a mempool handler that is useful for pipelining apps, where
the mempool cache doesn't really work - example, where we have one
core doing rx (and alloc), and another core doing Tx (and return).
In such a case, the mempool ring simply cycles through all the mbufs,
resulting in a LLC miss on every mbuf allocated when the number of
mbufs is large. A stack (LIFO) recycles buffers more effectively in
this case.

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>