[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.*}

JdbcToArrowUtils::updateVector(VarCharVector, String, boolean, int) から始め、NullableVarCharHolder がバッファーをどのように割り当てるかを確認します。JdbcToArrowTest::testJdbcToArrowValues() を確認し、issue に示されている allocator メモリのアサーションを追加します。root を閉じた後に、割り当て済みメモリがゼロの状態でテストがパスすれば作業完了です。

索引モデルが issue の本文から書いたものです。