Elastic bulk actions: flush after 20 docs

event with 1Mb we're still seeing errors related to the request size. Restrict the pending actions to at most 20. Given that our max doc size is around 4mb this should result in a request below 100Mb. This is far from ideal but might perhaps help to keep the pipeline running while a better workaround is found.

Bug: T353430 Change-Id: I305ce84637a3a971dd3a0e62fec9a3e246c1e497