Reduce CoordinatorServer memory footprint for kv snapshot

### Search before asking

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

### Description

Currently, CoordinatorServer will maintain the kv snapshots in memory to delete unused kv snapshot file via reference count which require to maintail all kv files(represent by `KvFileHandle`) in memory which is memory cost. We can reduce memory footprint by following ways:

### Willingness to contribute

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

Start by locating CoordinatorServer and KvFileHandle and tracing how kv snapshots are retained and deleted through reference counting. Review the existing snapshot cleanup path; done means unused kv snapshot files are still deleted correctly while CoordinatorServer holds less snapshot state in memory.

Written by the indexing model from the issue text.