Chunk server: fix rare failure uncovered by the endurance test with failure simulation when the unstable chunk is discarded due to version change coming at the moment when writes are still in flight. Most likely I forgot to remove this condition when I re-implemented (a few years ago) version change to handle this case by executing chunk header header write and rename after write completion. For now add parameter to turn this condition off by default, by allowing version version change with writes in flight in order to prevent erroneously discarding unstable chunk on re-connect.