mpi/pmix: set of optimizations for the _ucx_progress

- Avoid extra ucx_worker_progress call
- Avoid extra initialization in _ucx_progress call
The effect on ping-pong latency test (~500 ns improvement):
size    UCXv6	UCXv7
1       19.2    18.8
2       19.2    18.7
4       19.2    18.7
8       19.6    19.2
16      19.9    19.3
32      19.8    19.4
64      20.1    19.5
128     20.5    20.2
256     21.0    20.4
512     21.1    20.6
1024    20.9    20.5
2048    22.1    21.6
4096    25.3    24.6
8192    28.5    27.7
16384   31.2    30.7
32768   37.0    36.1
65536   48.1    47.8
131072  72.6    72.2
262144  2104.7  2229.3
524288  2722.0  2817.8
1048576 3756.2  3693.1
2097152 6206.3  6148.5
4194304 10281.3 10230.7

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