set the key of all mpool_tree_item objects, so they can be retrieved
in mpool_base_free and then returned back to the
mca_mpool_base_tree_item_free_list free list.
Refs. open-mpi/ompi#4567
Thanks Philip Blakely for the bug report.
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
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