Context:
As a follow-up from https://github.com/camunda/camunda-optimize/pull/2225#discussion_r442889027 it's worth replacing the atomic get's performed currently to get the next batch of instances to cleanup with an overall scroll context that would allow us to not refresh immediatly on every delete request as it's currently done.

AT:

• use scroll context to page through the instances to cleanup
• remove refresh.immediate from the delete bulks in processInstanceWriter/variableUpdateWriter