mempool: support handler operations

mempool: support handler operations

Until now, the objects stored in a mempool were internally stored in a
ring. This patch introduces the possibility to register external handlers
replacing the ring.

The default behavior remains unchanged, but calling the new function
rte_mempool_set_ops_byname() right after rte_mempool_create_empty() allows
the user to change the handler that will be used when populating
the mempool.

This patch also adds a set of default ops (function callbacks) based
on rte_ring.

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