code: public <T> Future<T> submit(SQLCallable<T> callable){
          return this.submitTaskToQueue(new SQLQueueCallable<T>(db, callable));
      }
docs: Submits a database task for execution
  @param callable The task to be performed
  @param <T> The type of object that is returned from the task
  @throws RejectedExecutionException Thrown when the queue has been shutdown
  @return Future representing the task to be executed.