RE: Slot's restart_lsn may point to removed WAL segment after hard restart unexpectedly

Dear Alexander,

> Thank you! All of these totally make sense. The updated patch is attached.

Thanks for the update. I found another point.

```
-# Another 2M rows; that's about 260MB (~20 segments) worth of WAL.
+# Another 50K rows; that's about 86MB (~5 segments) worth of WAL.
$node->safe_psql('postgres',
- q{insert into t (b) select md5(i::text) from generate_series(1,1000000) s(i)}
+ q{insert into t (b) select repeat(md5(i::text),50) from generate_series(1,50000) s(i)}
);
```

I think a perl function advance_wal() can be used instead of doing actual INSERT
command because no one refers the replicated result. Same thing can be said in
046/047.

Best regards,
Hayato Kuroda
FUJITSU LIMITED