When set, and when USING_EPHEMERAL_DISKS environment variable is set to true, forces all in-progress segments to be closed and uploaded to bucket, and also forces a write (and upload) of global snapshot during shutdown.

When not set, this avoids the extra work and thus time shutting down from flushing very recent segments, as those can then be resumed on next boot, assuming that next boot continues on the same Kafka epoch. When set to the default, allows faster shutdown.