talloc: Add a separate pool size

talloc: Add a separate pool size

This is necessary to allow talloc pools to be objects on their own. It
is an incompatible change in the sense that talloc_get_size(pool) now
returns 0 instead of the pool size. When the talloc_pooled_object()
call is added, this will start to make sense again.

Maybe we should add a talloc_pool_size call? Or is that overkill?

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>