mpi/pmix: Rework verification code on the fastpath

Effect on the ping-pong latency test:

size    UCXv3   UCXv4
1       24      20
2       22      20
4       23      20
8       30      21
16      27      21
32      22      21
64      22      22
128     29      22
256     24      22
512     26      23
1024    24      23
2048    27      24
4096    33      27
8192    31      30
16384   32      33
32768   58      38
65536   56      51
131072  79      76
262144  2258    2231
524288  3044    2868
1048576 3899    3749
2097152 6328    6210
4194304 10828   10641

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