Refactoring on the Anserini end to enable cleaner Pyserini bindings

[lintool]

• batch_search in dense searchers - changed method signature to take queries, then qids - to be consistent with SimpleSearcher and SimpleImpactSearcher
• dense searchers: refactor ThreadPoolExecutor to use try-with-resources, see Refactor ThreadPoolExecutor to use try-with-resources #2579

[codecov]

Codecov Report

Attention: Patch coverage is 68.62745% with 32 lines in your changes missing coverage. Please review.

Project coverage is 67.17%. Comparing base (9744463) to head (43be5d7).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...ain/java/io/anserini/search/FlatDenseSearcher.java	65.51%	7 Missing and 3 partials ⚠️
...ain/java/io/anserini/search/HnswDenseSearcher.java	65.51%	7 Missing and 3 partials ⚠️
...java/io/anserini/search/InvertedDenseSearcher.java	62.50%	6 Missing and 3 partials ⚠️
...ava/io/anserini/search/SearchFlatDenseVectors.java	87.50%	1 Missing ⚠️
...ava/io/anserini/search/SearchHnswDenseVectors.java	87.50%	1 Missing ⚠️
...io/anserini/search/SearchInvertedDenseVectors.java	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2584      +/-   ##
============================================
- Coverage     67.18%   67.17%   -0.02%     
+ Complexity     1480     1479       -1     
============================================
  Files           219      219              
  Lines         12700    12700              
  Branches       1530     1530              
============================================
- Hits           8533     8531       -2     
- Misses         3640     3641       +1     
- Partials        527      528       +1     

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[lintool]

@MXueguang this is ready for review. The code coverage check is pointing at lines that are difficult to test, so I'm going to punt on that now.

Running regressions - all seems good so far. Will circle back and confirm but CR can proceed in parallel.