Improve logging for lake tiering

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Description

Just written out some point in here, may add more in the future

1. make `LOG.info("Last committed lake table snapshot info is:{}", lakeSnapshotInfo);` as debug to avoid to many lgos
2. logging about assign which reader which split
3. logging commit status in commit operator

more

### Willingness to contribute

- [ ] I'm willing to submit a PR!