[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.*}

Beginne mit der UnionTest-Reproduktion und verfolge ArrowStreamReader.loadNextBatch über ArrowReader.loadRecordBatch und VectorLoader.load bis zu der Stelle, an der die Ausnahme gemeldet wird. Vergleiche die Ausgabe von ArrowStreamWriter und die Behandlung von UnionVector für die sparse union und überprüfe anschließend, dass der Reader den record batch ohne den nodes-and-buffers-Fehler verarbeitet.

Vom Indexierungsmodell aus dem Issue-Text verfasst.