Kafka premature flush when reaching end of partition

When a consumer gets to the end of topic flush happens w/o respect of `stream_flush_interval_ms` (that can lead to more frequent flushes of smaller blocks).

Discovered during testing of https://github.com/ClickHouse/ClickHouse/pull/11149