[Java] Arrow Java can't read union vector from ArrowStreamReader written by its own bugs

When writing union data using  ArrowStreamWriter in java, I can't read it back using ArrowStreamReader in java. The exception is:
> Exception in thread "main" java.lang.IllegalArgumentException: not all nodes and buffers were consumed. nodes: [ArrowFieldNode [length=100, nullCount=0]] buffers: [ArrowBuf[14], udle: [7 104..117], ArrowBuf[15], udle: [7 120..520]]
> at org.apache.arrow.vector.VectorLoader.load(VectorLoader.java:64)
> at org.apache.arrow.vector.ipc.ArrowReader.loadRecordBatch(ArrowReader.java:219)
> at org.apache.arrow.vector.ipc.ArrowStreamReader.loadNextBatch(ArrowStreamReader.java:121)
The code to reproduce this exception is:

 
```java

import org.apache.arrow.memory.RootAllocator;
import org.apache.arrow.vector.FieldVector;
import org.apache.arrow.vector.VectorSchemaRoot;
import org.apache.arrow.vector.complex.UnionVector;
import org.apache.arrow.vector.dictionary.DictionaryProvider;
import org.apache.arrow.vector.holders.NullableIntHolder;
import org.apache.arrow.vector.ipc.ArrowStreamReader;
import org.apache.arrow.vector.ipc.ArrowStreamWriter;
import org.apache.arrow.vector.types.UnionMode;
import org.apache.arrow.vector.types.pojo.ArrowType;
import org.apache.arrow.vector.types.pojo.Field;
import org.apache.arrow.vector.types.pojo.FieldType;
import org.apache.arrow.vector.types.pojo.Schema;

import java.io.ByteArrayInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Collections;
import java.util.List;

public class UnionTest {

public static void writeUnionBatch(OutputStream os) throws IOException {
int[] typeIds = new int[]{ArrowType.ArrowTypeID.Int.ordinal()};
ArrowType.Union union = new ArrowType.Union(UnionMode.Sparse, typeIds);
List childList = Collections.singletonList(
new Field("s1", FieldType.nullable(new ArrowType.Int(32, true)), null)
);
Field field = new Field("f1", FieldType.nullable(union), childList);
List fields = Collections.singletonList(field);
Schema schema = new Schema(fields);
VectorSchemaRoot root = VectorSchemaRoot.create(schema, new RootAllocator(Integer.MAX_VALUE));
DictionaryProvider.MapDictionaryProvider provider = new DictionaryProvider.MapDictionaryProvider();
ArrowStreamWriter writer = new ArrowStreamWriter(root, provider, os);
writer.start();
for (int i = 0; i < 2; i++) {
root.setRowCount(100);
List vectors = root.getFieldVectors();
UnionVector vector = (UnionVector) vectors.get(0);
fillVector(vector, 100);
for (int j = 0; j < 100; j++) {
if (!vector.isNull(j)) {
System.out.println(vector.getObject(j));
}
}
writer.writeBatch();
}
writer.end();
writer.close();
}

private static void fillVector(UnionVector vector, int batchSize) {
vector.setInitialCapacity(batchSize);
vector.allocateNew();
for (int i = 0; i < batchSize; i++) {
NullableIntHolder intHolder = new NullableIntHolder();
intHolder.isSet = 1;
intHolder.value = i;
vector.setSafe(i, intHolder);
}
vector.setValueCount(batchSize);
}

public static void main(String[] args) throws IOException {
try(FileOutputStream fos = new FileOutputStream("result/union.arrow")) {
writeUnionBatch(fos);
System.out.println("write succeed");
fos.flush();
}

RootAllocator allocator = new RootAllocator(1000000000);
ByteArrayInputStream in = new ByteArrayInputStream(Files.readAllBytes(Paths.get("result/union.arrow")));
ArrowStreamReader reader = new ArrowStreamReader(in, allocator);
reader.loadNextBatch();
}
}

```
And it can't read union data generated by python, as is reported in https://issues.apache.org/jira/browse/ARROW-1692. 

It seems strange arrow java can't read union data generated by its own. Is there any format gap between arrow java UnionVector write and read?

**Environment**: Mac OS 10.13.6, Arrow 0.13.0, JDK8
**Reporter**: [Shawn Yang](https://issues.apache.org/jira/browse/ARROW-5231) / @chaokunyang

_{**Note**: *This issue was originally created as [ARROW-5231](https://issues.apache.org/jira/browse/ARROW-5231). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

UnionTest の再現から始め、例外が報告される箇所まで ArrowReader.loadRecordBatch と VectorLoader.load を経由して ArrowStreamReader.loadNextBatch を追跡します。ArrowStreamWriter の出力と sparse union に対する UnionVector の処理を比較し、その後、reader が nodes-and-buffers エラーなしで record batch を消費することを確認します。

索引モデルが issue の本文から書いたものです。