Fix diskbbq flush logic

[benwtrent]

I accidentally broke recall on flush by allowing vectors to be double quantized. Additionally, we shouldn't use the first vector as a centroid, this can harm recall significantly when there is just one centroid.

recall before this change:

index_name                             index_type  num_docs  index_time(ms)  force_merge_time(ms)  num_segments
-------------------------------------  ----------  --------  --------------  --------------------  ------------
corpus-dbpedia-entity-E5-small-0.fvec         ivf   1000000           25820                     0            14
corpus-dbpedia-entity-E5-small-0.fvec         ivf   1000000               0                 41693             0

index_name                             index_type  n_probe  latency(ms)  net_cpu_time(ms)  avg_cpu_count     QPS  recall    visited  filter_selectivity
-------------------------------------  ----------  -------  -----------  ----------------  -------------  ------  ------  ---------  ------------------
corpus-dbpedia-entity-E5-small-0.fvec         ivf       50        13.05              0.00           0.00   76.61    0.63  285267.44                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      150        31.92              0.00           0.00   31.33    0.68  629033.22                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      200        34.79              0.00           0.00   28.74    0.69  679699.13                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      500        39.40              0.00           0.00   25.38    0.71  794375.05                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf     1000        45.99              0.00           0.00   21.74    0.72  940493.52                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf       50         1.52              0.00           0.00  655.74    0.74   24201.82                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      150         2.94              0.00           0.00  340.43    0.85   67943.31                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      200         3.81              0.00           0.00  262.81    0.87   89575.99                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      500         7.67              0.00           0.00  130.38    0.93  213586.44                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf     1000        14.85              0.00           0.00   67.33    0.96  402628.11                1.00

With this fix:

index_name                             index_type  num_docs  index_time(ms)  force_merge_time(ms)  num_segments
-------------------------------------  ----------  --------  --------------  --------------------  ------------
corpus-dbpedia-entity-E5-small-0.fvec         ivf   1000000           25304                     0            15
corpus-dbpedia-entity-E5-small-0.fvec         ivf   1000000               0                 42110             0

index_name                             index_type  n_probe  latency(ms)  net_cpu_time(ms)  avg_cpu_count     QPS  recall    visited  filter_selectivity
-------------------------------------  ----------  -------  -----------  ----------------  -------------  ------  ------  ---------  ------------------
corpus-dbpedia-entity-E5-small-0.fvec         ivf       50        12.63              0.00           0.00   79.18    0.89  285527.22                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      150        32.49              0.00           0.00   30.77    0.94  619783.37                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      200        35.46              0.00           0.00   28.20    0.95  667903.47                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      500        40.38              0.00           0.00   24.76    0.97  781959.74                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf     1000        48.62              0.00           0.00   20.57    0.98  931017.40                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf       50         1.55              0.00           0.00  643.09    0.74   23595.57                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      150         2.98              0.00           0.00  335.29    0.85   66299.43                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      200         3.81              0.00           0.00  262.64    0.87   87416.15                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf      500         8.80              0.00           0.00  113.64    0.93  209061.37                1.00
corpus-dbpedia-entity-E5-small-0.fvec         ivf     1000        16.18              0.00           0.00   61.81    0.96  394906.29                1.00

[elasticsearchmachine]

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

[john-wagster]

lgtm