Improve DirectIO prefetch strategy in Lucene99 flat vector format

[iverase]

This approach takes a more aggressive prefetching strategy by prefetching the next bulk before executing the current one.

Testing shown an improvement:

before:

index_name       index_type  visit_percentage(%)  latency(ms)  net_cpu_time(ms)  avg_cpu_count    QPS  recall    visited  filter_selectivity
---------------  ----------  -------------------  -----------  ----------------  -------------  -----  ------  ---------  ------------------  
wiki1024en.docs         ivf                 0.25        14.74              0.00           0.00  67.84    0.73    5828.14                1.00
wiki1024en.docs         ivf                 0.50        16.03              0.00           0.00  62.38    0.83   10854.43                1.00
wiki1024en.docs         ivf                 0.75        14.74              0.00           0.00  67.84    0.88   15880.75                1.00
wiki1024en.docs         ivf                 1.00        14.84              0.00           0.00  67.39    0.91   20883.03                1.00
wiki1024en.docs         ivf                 1.50        15.05              0.00           0.00  66.45    0.94   30841.79                1.00
wiki1024en.docs         ivf                 2.00        15.21              0.00           0.00  65.75    0.95   40852.71                1.00
wiki1024en.docs         ivf                 2.50        15.41              0.00           0.00  64.89    0.96   50842.37                1.00
wiki1024en.docs         ivf                 0.00        19.39              0.00           0.00  51.57    0.99  180853.42                1.00

after:

index_name       index_type  visit_percentage(%)  latency(ms)  net_cpu_time(ms)  avg_cpu_count    QPS  recall    visited  filter_selectivity
---------------  ----------  -------------------  -----------  ----------------  -------------  -----  ------  ---------  ------------------  
wiki1024en.docs         ivf                 0.25        11.74              0.00           0.00  85.18    0.73    5828.14                1.00
wiki1024en.docs         ivf                 0.50        11.82              0.00           0.00  84.60    0.83   10854.43                1.00
wiki1024en.docs         ivf                 0.75        11.51              0.00           0.00  86.88    0.88   15880.75                1.00
wiki1024en.docs         ivf                 1.00        11.13              0.00           0.00  89.85    0.91   20883.03                1.00
wiki1024en.docs         ivf                 1.50        11.65              0.00           0.00  85.84    0.94   30841.79                1.00
wiki1024en.docs         ivf                 2.00        11.74              0.00           0.00  85.18    0.95   40852.71                1.00
wiki1024en.docs         ivf                 2.50        12.33              0.00           0.00  81.10    0.96   50842.37                1.00
wiki1024en.docs         ivf                 0.00        15.98              0.00           0.00  62.58    0.99  180853.42                1.00

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)