Avoid unnecessary StringInfo allocation in tablesync COPY buffer

Hi,

I found this issue while reviewing the patch [1] and was suggested use a separate thread for the issue.

In tablesync.c, copy_table() currently does:
```
copybuf = makeStringInfo();
```

But copybuf is only used by copy_read_data(), and there it's really just acting as a small state holder for data, len, and cursor, rather than as a normal growable StringInfo. That means we do not need to allocate a StringInfo object or its backing buffer at all.

It would be cleaner to use a plain StringInfoData and simply reinitialize or zero it in copy_table(). See the attached patch for the proposed change.

David Rowley has made several cleanup changes in this area to prefer stack-allocated StringInfoData, for example a63bbc811d41b3567eb37fe2636e660a852dbbf2. This change seems consistent with that direction.

[1] https://postgr.es/m/CAOzEurQKuy3RiPkd=25PEwEzaqHuGvEOf=X7vaVzhgNjaukYzA@mail.gmail.com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/