mpi/pmix: address UCX perf. issues (custom list implementation)

Analysis indicates that slurm's `List` introduces significant overhead
(up to 7 us for append operation). More lightweight and less generic
version of a list container was developed for PMIx plugin with performance
considerations in mind.

Internal pingpong latency test results demonstrating improvements:

size    UCXv1   UCX v2
1       46      30
2       56      30
4       72      30
8       85      30
16      93      30
32      122     31
64      120     31
128     126     31
256     137     32
512     155     33
1024    142     33
2048    153     34
4096    153     37
8192    170     42
16384   173     45
32768   177     52
65536   218     64
131072  236     88
262144  2735    2248
524288  3254    2879
1048576 4303    3770
2097152 6917    6234
4194304 11579   10473

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