ARROW-6234: [Java] ListVector hashCode() is not correct

Related to [ARROW-6234](https://issues.apache.org/jira/browse/ARROW-6234).
Current implement is not correct:
for (int i = start; i < end; i++) {
  hash = 31 * vector.hashCode(i);
}
Should be something like:
hash = 31 * hash + vector.hashCode(i);

Closes apache#5082 from tianchen92/ARROW-6234 and squashes the following commits:

9ad7a7e <tianchen92> Merge branch 'master' into ARROW-6234
ff2b01f <tianchen92> Merge branch 'master' into ARROW-6234
0bb79e2 <tianchen> ARROW-6234:  ListVector hashCode() is not correct

Lead-authored-by: tianchen <niki.lj@alibaba-inc.com>
Co-authored-by: tianchen92 <niki.lj@alibaba-inc.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>

Author: tianchen92

java/vector/src/main/java/org/apache/arrow/vector/complex/ListVector.java

@@ -28,6 +28,7 @@
 import org.apache.arrow.memory.BaseAllocator;
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.memory.OutOfMemoryException;
+import org.apache.arrow.memory.util.ByteFunctionHelpers;
 import org.apache.arrow.util.Preconditions;
 import org.apache.arrow.vector.AddOrGetResult;
 import org.apache.arrow.vector.BitVectorHelper;
@@ -425,7 +426,7 @@ public int hashCode(int index) {
     final int start = offsetBuffer.getInt(index * OFFSET_WIDTH);
     final int end = offsetBuffer.getInt((index + 1) * OFFSET_WIDTH);
     for (int i = start; i < end; i++) {
-      hash = 31 * vector.hashCode(i);
+      hash = ByteFunctionHelpers.comebineHash(hash, vector.hashCode(i));
     }
     return hash;
   }

java/vector/src/main/java/org/apache/arrow/vector/complex/NonNullableStructVector.java

@@ -26,6 +26,7 @@
 import java.util.Map;
 
 import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.memory.util.ByteFunctionHelpers;
 import org.apache.arrow.util.Preconditions;
 import org.apache.arrow.vector.DensityAwareVector;
 import org.apache.arrow.vector.FieldVector;
@@ -301,7 +302,7 @@ public int hashCode(int index) {
     for (String child : getChildFieldNames()) {
       ValueVector v = getChild(child);
       if (v != null && index < v.getValueCount()) {
-        hash += 31 * hash + v.hashCode(index);
+        hash = ByteFunctionHelpers.comebineHash(hash, v.hashCode(index));
       }
     }
     return hash;