[JAVA] Leak in JdbcToArrowUtils

`JdbcToArrowUtils::updateVector(VarCharVector, String, boolean, int)` does not release the memory that it allocates for the `NullableVarCharHolder`. This can be verified by changing the first lines of `JdbcToArrowTest::testJdbcToArrowValues()` to the following:
```java

RootAllocator allocator = new RootAllocator(Integer.MAX_VALUE);
VectorSchemaRoot root = JdbcToArrow.sqlToArrow(conn, table.getQuery(), allocator, Calendar.getInstance());
testDataSets(root);
root.close();
assertEquals(allocator.getAllocatedMemory(), 0);
```
 

The leak can be fixed by closing the buffer in `updateVector`. However, I would propose to not use the `NullableVarCharHolder` in the first place because it creates an unnecessary copy of the string. Instead, I would use `BaseVariableWidthVector::setSafe(int index, byte[] value)` and `BaseVariableWidthVector::setNull(int index)` directly.

In addition, I would propose to check for leaks in JdbcToArrowTest.

I am willing to create a patch, so please let me know what approach is preferred.

**Reporter**: [Johannes Luong](https://issues.apache.org/jira/browse/ARROW-5536) / @jmaschad

_{**Note**: *This issue was originally created as [ARROW-5536](https://issues.apache.org/jira/browse/ARROW-5536). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Beginne in JdbcToArrowUtils::updateVector(VarCharVector, String, boolean, int) und untersuche, wie NullableVarCharHolder seinen Puffer alloziert. Überprüfe JdbcToArrowTest::testJdbcToArrowValues() und füge die im Issue gezeigte Assertion für den Allocator-Speicher hinzu; die Arbeit ist abgeschlossen, wenn der Test nach dem Schließen des Root mit null alloziertem Speicher besteht.

Vom Indexierungsmodell aus dem Issue-Text verfasst.