spin-off from #61587

If a transform is used with date_histogram transform creates and updates intermediate buckets, because checkpointing is not aligned with the interval in a the date histogram. For usecases with lot's of data and/or low intervals, this creates extra load, because upserts (delete + create) are expensive.

If checkpointing was aligned with bucket boundaries, upserts could be avoided.

Pro's

• better performance, no deletes
• destination could be a data stream
• destination could be managed with ILM (no duplicates, see Continuous transform with date histogram generates duplicates after index rollover #61587)
• avoid confusion with intermediate results in upstream usage