mempool: add structure for object headers

mempool: add structure for object headers

Each object stored in mempools are prefixed by a header, allowing for
instance to retrieve the mempool pointer from the object. When debug is
enabled, a cookie is also added in this header that helps to detect
corruptions and double-frees.

Introduce a structure that materializes the content of this header,
and will simplify future patches adding things in this header.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>