Discussion: Optimize `RollingFileWriter` by closing asynchronously

### Is your feature request related to a problem or challenge?

Reference: https://github.com/apache/iceberg-rust/pull/1547#discussion_r2229405980

When rolling over to a new file, instead of waiting on the current writer to close, we can spawn a thread to improve its efficiency. But we need to be careful about the max parallelism since too many writers or an over-sized buffer can exhaust the memory

### Describe the solution you'd like

_No response_

### Willingness to contribute

None

Start with the RollingFileWriter discussion linked from PR #1547, focusing on rollover behavior and the proposed asynchronous close. Determine how max parallelism and buffer size should be bounded, then define completion criteria that preserve memory safety while improving rollover efficiency.

Written by the indexing model from the issue text.