[server] Support time-based log segment rolling (segment.ms)

[fxbing]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Fluss currently rolls log segments only when size, index, or relative-offset limits are reached. Because retention and tiering skip the active segment, a low-throughput bucket can keep data in the active segment indefinitely.

Solution

Add an optional segment.ms-like threshold to the existing append-time roll check. Use the replicated record-batch commitTimestamp so leader and follower choose the same segment boundary, and keep the option disabled by default.

Willingness to contribute

• I'm willing to submit a PR!