Replace regular array to array of structs to get rid of false sharing (#10892)

There are a lot of false sharing cache misses in line 4013 inside getIOPendingCount function.
The reason is that elements of io_threads_pending array access the same cache line from different
threads, so it is better to represent it as an array of structures with fields aligned to cache line size.
This change should improve performance (in particular, it affects the latency metric, we saw up to 3% improvement).