initial implementation for hnsw

[MXueguang]

indexed with 25 threads and default HNSW config
indexing time 12:20:00
efS 10, QPS 716, MRR 0.2376
efS 100, QPS 332, MRR 0.3103
efS 1000, QPS 58, MRR 0.3275

indexed with 1 thread and default HNSW config
indexing time 17:26:00
efS 10, QPS 762, MRR 0.2199
efS 100, QPS 331, MRR 0.3060
efS 1000, QPS 62, MRR 0.3266

multi-thread search works well

indexing seems very slow currently, (slower than index using ES)

[lintool]

Initial comments...

[codecov-commenter]

Codecov Report

Merging #1955 (2b400e2) into master (cc6337c) will decrease coverage by 2.53%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #1955      +/-   ##
============================================
- Coverage     60.14%   57.60%   -2.54%     
  Complexity     1061     1061              
============================================
  Files           178      187       +9     
  Lines          9903    10339     +436     
  Branches       1371     1422      +51     
============================================
  Hits           5956     5956              
- Misses         3441     3877     +436     
  Partials        506      506              

Impacted Files	Coverage Δ
.../java/io/anserini/collection/VectorCollection.java	0.00% <0.00%> (ø)
src/main/java/io/anserini/index/IndexArgs.java	100.00% <ø> (ø)
...c/main/java/io/anserini/index/IndexVectorArgs.java	0.00% <0.00%> (ø)
.../java/io/anserini/index/IndexVectorCollection.java	0.00% <0.00%> (ø)
...index/generator/LuceneVectorDocumentGenerator.java	0.00% <0.00%> (ø)
...main/java/io/anserini/search/SearchVectorArgs.java	0.00% <0.00%> (ø)
...ava/io/anserini/search/SearchVectorCollection.java	0.00% <0.00%> (ø)
...io/anserini/search/query/VectorQueryGenerator.java	0.00% <0.00%> (ø)
...i/search/topicreader/JsonIntVectorTopicReader.java	0.00% <0.00%> (ø)
...earch/topicreader/JsonStringVectorTopicReader.java	0.00% <0.00%> (ø)

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[lintool]

Looks good for now, but we'll need to circle back to add tests later.