Flush Segments on a Schedule

Currently, a segment can be flushed based on the size, a number of rows, or time since creation. It would be great if there is a way to trigger a flush based on a (cron) schedule.

The motivation is that it can take minutes to flush a segment, during which Pinot stops consuming new messages and hence there would be a delay of minutes. This may totally throw the users off for time-sensitive use cases.

Start by locating Pinot's existing segment flush triggers for size, row count, and age. Determine how a cron-based trigger should interact with ongoing consumption and define tests for scheduled flushing and the reported ingestion delay; done means the schedule is configurable and the behavior is covered.

Written by the indexing model from the issue text.