[Feature] Control the background job thread nums of rocksdb

### Search before asking

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

### Motivation

Currently, we will create two background threads per bucket to do fluss/compaction which may cause controllable threads.

### Solution

We can control the background threads in server level. Thanksfully, Rocksdb allows us share a threads pool across rocksdbs.
See more in https://github.com/facebook/rocksdb/blob/3ef909248736eb80e7c6b3c7a19753c4dbe9d0c5/include/rocksdb/env.h#L465

### Anything else?

_No response_

### Willingness to contribute

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

Start by tracing where the server creates RocksDB instances and where the two background threads per bucket are configured. Read the linked RocksDB Env thread-pool documentation, then determine how a server-level shared pool can control those threads; done means the background thread count is configurable without creating a separate pool per bucket.

Written by the indexing model from the issue text.