[Java] Netty memory release

### Describe the bug, including details regarding any error messages, version, and platform.

Since when we use netty as my memory allocator manager, I found that direct memory is not released after vector is closed.
```
Field field = new Field("test", FieldType.nullable(new ArrowType.Utf8()), null);
RootAllocator allocator = new RootAllocator(Long.MAX_VALUE);
FieldVector vector = field.createVector(allocator);
for (int i = 0; i < 100000; i++){
((VarCharVector)vector).setSafe(i, "test".getBytes(StandardCharsets.UTF_8));
}

System.out.println("before close memory: " + usedDirectMemory() + " bytes"); //before close memory: 4194304 bytes
vector.close();
allocator.close();
System.out.println("after closed memory: " + usedDirectMemory() + " bytes"); //after closed memory: 4194304 bytes
```
I can address the problem that if the inner allocator `PooledByteBufAllocator` won't free the memory chunk , I am not sure if this is expected.

### Component(s)

Java

Comece com a reprodução em Java fornecida usando RootAllocator, VarCharVector e PooledByteBufAllocator; em seguida, rastreie o comportamento de fechamento do allocator e o gerenciamento de memória direta do Netty. Confirme se os 4194304 bytes restantes são um vazamento real ou um comportamento esperado de pooling, e documente ou teste o comportamento esperado resultante.

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