dblink_send_query sends a query to be executed
    asynchronously, that is, without immediately waiting for the result.
    There must not be an async query already in progress on the
    connection.
   
    After successfully dispatching an async query, completion status
    can be checked with dblink_is_busy, and the results
    are ultimately collected with dblink_get_result.
    It is also possible to attempt to cancel an active async query
    using dblink_cancel_query.