hash: add iterate function

hash: add iterate function

Since now rte_hash structure is private, a new function
has been added to let the user iterate through the hash table,
returning next key and data associated on each iteration,
plus the position where they were stored.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>