Skip heavy TreeSet opts for the first group in SearchGroup#merge

[gaobinlong]

Description

Similar to #15128, when merging groups across shards in SearchGroup#merge, we can update the first group in the min-heap in place when more competitive sort values are found.

Benchmark test on wikimedium10k:

TaskQPS baseline      StdDevQPS my_modified_version      StdDev                Pct diff
p-value
                    TermGroup10K      125.15      (7.3%)      128.31      (8.6%)    2.5% ( -12% -   19%) 0.317
                     TermGroup1M      130.04     (10.5%)      133.37     (11.6%)    2.6% ( -17% -   27%) 0.463
                    TermGroup100      156.47     (10.8%)      165.87      (9.7%)    6.0% ( -13% -   29%) 0.064

[gaobinlong]

Hi @benwtrent , could you help to review this PR? Thanks!

[jainankitk]

Looks fine to me!

[gaobinlong]

Thanks! @jainankitk