Implements recursive doubling algorithm for MPI_Iexscan.
The algorithm preserves order of operations so it can be used both
by commutative and non-commutative operations.
The MCA parameter 'coll_libnbc_iexscan_algorithm' was added for dynamic
algorithm selection.
Signed-off-by: Mikhail Kurnosov <mkurnosov@gmail.com>
(cherry picked from commit dfe203e167f5d8abc3b55226c6f17a468c9567dd)
Signed-off-by: Brian Barrett <bbarrett@amazon.com>
de5e435dee