[Question][pgwak] pgwal插件sql方式支持配置lsn，如果制定lsn所在日志文件已归档，是否仍能从制定lsn读数据

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### Description

目前仿照chunjun的插件，测试了一把，测试场景如下：
1. 不指定lsn，跑一段时间，然后停止，记录下slot中的restar_lsn,然后继续开始跑一段时间，此时slot的restar_lsn已发生多次更新
2. 停止任务，修改配置，制定lsn为上述记录的restar_lsn

结果：
并没有达到预期，从制定的lsn开始读数据，而是继续从上一次停止时slot中的restar_lsn开始的

疑问：
1. 是否支持从指定的lsn开始读
2. 或则什么情况下指定的lsn生效

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