[Tech Request]: avoid flushing appendable objects

### Is there an existing issue for the same tech request?

- [X] I have checked the existing issues.

### Does this tech request not affect user experience?

- [X] This tech request doesn't affect user experience.

### What would you like to be added ?

```Markdown
Currently, if there are 3 appendable objects: ObjA, ObjB, ObjC, we'll flush 3 objects and at least one more merged object. We can avoid flushing appendable objects and only flush one merged object. We can do several things in one transaction:
1. Merge ObjA, ObjB, ObjC into ObjD
2. Add a new column `commit_ts` in ObjD to record all committed ts from ObjA,ObjB,ObjC
2. Update the meta-location for ObjA, ObjB and ObjC with the meta-location of ObjD
```

### Why is this needed ?

_No response_

### Additional information

_No response_