Executor执行器中beforeImage和afterImage方法返回值只考虑了sql语句涉及一张表的情况，无法承接涉及多张表的SQL语句，如MultiUpdateExecutor

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这里的beforeImage只考虑了一张表，但像MultiExecutor这种可能涉及多张表，目前是在内部通过维护map结构实现的，返回值是null，语义上不太明确。可考虑beforeImage的抽象方法返回值定义为泛型，由各个子类根据实际情况承接不同的返回值

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