Clear PreparedStatement pool when connection is returned to the pool

With the current configuration option poolPreparedStatements true the statements are held open for the lifetime of the connection. This results in cursors being open and locks in the database for a long time, which could cause problems with administrative tasks in the database.

There should be an additional configuration clearStatementPoolOnReturn. When set to true, the pool will be cleared and the statements closed when the connection is returned to the pool. Default can be false to retain the current behaviour.

With this the application can still benefit from the Statement-cache in mass operations.