Allow customized ExportRecordWriters the opportunity to exceute code after the last commit is performed and before the database connection is closed.

Custom ExportRecordWriters currently have no way of executing code after the last commit is performed and before the connection is closed.
The close() method that is available is not suitable, as it is called prior to the last batch of rows are inserted into the database. Attempting to execute code after the close() method executes is not viable, as the database connection is unavailable at this point.

The motivation for this change is prompted by the need to perform database operations after all the data has been inserted into the export table. For example, to drop temporary database tables etc.