This extracts a super class out of the rollup indexer called the AsyncTwoPhaseIterator.
The implementor of it can define the query, transformation of the response,
indexing and the object to persist the position/state of the indexer.
The stats object used by the indexer to record progress is also now abstract, allowing
the implementation provide custom stats beyond what the indexer provides. It also
allows the implementation to decide how the stats are presented (leaves toXContent()
up to the implementation).
This should allow new projects to reuse the search-then-index persistent task that Rollup
uses, but without the restrictions/baggage of how Rollup has to work internally to
satisfy time-based rollups.
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