mpi/pmix: Further reduce _ucx_progress overhead

Put request that is completed already directly to the _rcv_complete
list.
Ping-pong latency effect:
size    UCXv8   UCXv9
1       17.7    17.8
2       17.7    17.7
4       17.9    17.7
8       18.3    18.1
16      18.4    18.3
32      18.3    18.3
64      19.0    18.8
128     19.4    19.3
256     19.8    19.5
512     19.9    19.7
1024    19.7    19.6
2048    20.9    20.7
4096    23.7    23.5
8192    27.2    27.2
16384   30.3    30.0
32768   35.5    35.3
65536   47.4    47.2
131072  71.4    71.2
262144  2197.0  2202.2
524288  2727.1  2745.2
1048576 3580.4  3577.7
2097152 6120.7  6125.0
4194304 10329.2 10265.2

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