[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

从使用 RootAllocator、VarCharVector 和 PooledByteBufAllocator 的 Java 复现开始，然后跟踪 allocator 的关闭行为以及 Netty 对直接内存的处理。确认剩余的 4194304 字节究竟是实际的泄漏，还是预期的池化行为，并记录或测试由此得到的预期行为。

由索引模型根据 Issue 内容生成。