MINOR: memory: add a callback function to create a pool

MINOR: memory: add a callback function to create a pool

The new function create_pool_callback() takes 3 args including the
return pointer, and creates a pool with the specified name and size.
In case of allocation error, it emits an error message and returns.

The new macro REGISTER_POOL() registers a callback using this function
and will be usable to request some pools creation and guarantee that
the allocation will be checked. An even simpler approach is to use
DECLARE_POOL() and DECLARE_STATIC_POOL() which declare and register
the pool.