Introduce totalHitsThresholdPerGroup parameter to TopGroupsCollector and TopGroupsCollectorManager

[gaobinlong]

Description

Resolve a TODO in TopGroupsCollector.

Add totalHitsThresholdPerGroup parameter to TopGroupsCollector and TopGroupsCollectorManager. When set to a value less than Integer.MAX_VALUE, per-group hit counts may be reported as TotalHits.Relation.GREATER_THAN_OR_EQUAL_TO once totalHitsThresholdPerGroup hits have been collected, saving the cost of exact counting. Defaults to true (exact counts).

Benchark on wikimedium1m with set totalHitsThresholdPerGroup to maxDocsPerGroup(10):

                        TaskQPS baseline      StdDevQPS my_modified_version      StdDev                Pct diff p-value
                TermGroup100      109.06     (29.0%)      110.12     (31.7%)    1.0% ( -46% -   86%) 0.903
                 TermGroup1M       83.43     (21.7%)       85.24     (21.9%)    2.2% ( -34% -   58%) 0.705
                TermGroup10K      119.36     (11.6%)      123.38     (12.0%)    3.4% ( -18% -   30%) 0.277

[gaobinlong]

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