Update latest and stable offset if it was in stabilizing state

[akrambek]

Description

This is a current understanding of the issue

zilla: offset <= stable <= latest
which means offset <= stable, stable <= latest, offset <= latest
kafka: offset < stable <= high water mark
which means offset < stable, stable <= high water mark, offset < high water mark

Due to this invariant sometimes we may get into a state where the fix to send a flush when stableOffset or latestOffset has advanced (with no records) after records have been sent or caught up to the latest offset so at least one FLUSH was already sent.

Fixes #666