mpi/pmix: Add collectives evaluation tool

Add new performance debugging feature to pmix plugin that allows to
measure plain collectives performance on selected message sizes.
This functionality is similar to a ping-pong feature.

Performance results for existing point-to-point modes:

size    sapi            dtcp            ducx
1       0.002319442     0.000492334     0.000135243
2       0.002318223     0.000453552     0.000137356
4       0.00227046      0.00045832      0.000137117
8       0.002342675     0.000463539     0.000136455
16      0.00235131      0.000481208     0.00013619
32      0.002333058     0.000562986     0.000140756
64      0.002456691     0.000883791     0.000142574
128     0.002953556     0.001326429     0.000142336
256     0.003892236     0.002324766     0.000161224
512     0.006044123     0.004371988     0.000177675
1024    0.010324001     0.008485476     0.000224325
2048    0.018556118     0.016488896     0.000347243
4096    0.035331223     0.032744778     0.000481764
8192    0.06957123      0.065519465     0.001194106
16384   0.137925333     0.130130662     0.002544668
32768   0.272100422     0.259290563     0.009916888
65536   0.543431362     0.486692217     0.012841119

Signed-off-by: Artem Polyakov <artpol84@gmail.com>