Make zoekt index pending_deletion state transition event driven

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The following discussion from !165956 (merged) should be addressed:

• @johnmason started a discussion: (+1 comment)

  suggestion (non-blocking): I'd prefer we keep scheduling service readonly and "schedule" read/write changes in the context of another worker. The eviction task is quite complex already. That being said, we are already doing destroy_all here so this is a simple iteration. We can back and refactor this to use event store at another time.