Switch from syncronized to concurrent map for pq codebook (#518)

* Switch from syncronized to concurrent map for pq codebook

Signed-off-by: Jake Luciani <jake@datastax.com>

* Remove synchronized

---------

Signed-off-by: Jake Luciani <jake@datastax.com>

Author: Jake Luciani

jvector-base/src/main/java/io/github/jbellis/jvector/quantization/ImmutablePQVectors.java

@@ -22,8 +22,8 @@
 import io.github.jbellis.jvector.vector.types.ByteSequence;
 import io.github.jbellis.jvector.vector.types.VectorFloat;
 
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 public class ImmutablePQVectors extends PQVectors {
     private final int vectorCount;
@@ -41,7 +41,7 @@ public ImmutablePQVectors(ProductQuantization pq, ByteSequence<?>[] compressedDa
         this.compressedDataChunks = compressedDataChunks;
         this.vectorCount = vectorCount;
         this.vectorsPerChunk = vectorsPerChunk;
-        this.codebookPartialSumsMap = new HashMap<>();
+        this.codebookPartialSumsMap = new ConcurrentHashMap<>();
     }
 
     @Override
@@ -54,7 +54,7 @@ public int count() {
         return vectorCount;
     }
 
-    private synchronized VectorFloat<?> getOrCreateCodebookPartialSums(VectorSimilarityFunction vsf) {
+    private VectorFloat<?> getOrCreateCodebookPartialSums(VectorSimilarityFunction vsf) {
         return codebookPartialSumsMap.computeIfAbsent(vsf, pq::createCodebookPartialSums);
     }