[Java] JDBC to Arrow converter iterator should reuse the same VectorSchemaRoot

When originally reviewing the code I suggested a new VectorSchemaRoot on each call to the iterator.  After further discussions on the mailing list, it seems that this is an anit-pattern for working with VectorSchemaRoot, we should update the code to update a single VectorSchemaRoot.

After this change it should be easier to use JDBC converter with other components of the library (i.e. filewriter) which also make use of a single VectorSchemaRoot.

**Reporter**: [Micah Kornfield](https://issues.apache.org/jira/browse/ARROW-9614) / @emkornfield

_{**Note**: *This issue was originally created as [ARROW-9614](https://issues.apache.org/jira/browse/ARROW-9614). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by locating the Java JDBC converter's iterator and compare its VectorSchemaRoot handling with the filewriter components mentioned in the issue. Confirm how the iterator currently creates roots and identify any related tests. Done means the converter reuses one VectorSchemaRoot and works with components that expect a single root.

Written by the indexing model from the issue text.