implement throttling in async-indexer used by rollup and transform. The added
docs_per_second parameter is used to calculate a delay before the next
search request is send. With re-throttle its possible to change the parameter
at runtime. When stopping a running job, its ensured that despite throttling
the indexer stops in reasonable time. This change contains the groundwork, but
does not expose the new functionality.
relates #54862
backport: #55011