mpi/pmix: Immediate send when possible.

Latency results:
1       0.000079
2       0.000082
4       0.000078
8       0.000084
16      0.000084
32      0.000083
64      0.000091
128     0.000100
256     0.000113
512     0.000125
1024    0.000177
2048    0.000427
4096    0.000514
8192    0.000522
16384   0.000590
32768   0.000872
65536   0.001745
131072  0.002758
262144  0.004833
524288  0.009263
1048576 0.018100
2097152 0.036140
4194304 0.071862

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