mempool: allow user-owned cache

mempool: allow user-owned cache

The mempool cache is only available to EAL threads as a per-lcore
resource. Change this so that the user can create and provide their own
cache on mempool get and put operations. This works with non-EAL threads
too. This commit introduces the new API calls:

    rte_mempool_cache_create(size, socket_id)
    rte_mempool_cache_free(cache)
    rte_mempool_cache_flush(cache, mp)
    rte_mempool_default_cache(mp, lcore_id)

Changes the API calls:

    rte_mempool_generic_put(mp, obj_table, n, cache, flags)
    rte_mempool_generic_get(mp, obj_table, n, cache, flags)

The cache-oblivious API calls use the per-lcore default local cache.

Signed-off-by: Lazaros Koromilas <l@nofutznetworks.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>