Context:
In order to avoid rerunning/restarting costly redundant reindex operations when resuming an upgrade it would be desirable to recognize an already running previously started operation and wait for it to finish rather then submitting the same operation again.

AT:

• already running reindex operations are detected and the upgrade waits for their completion instead of rerunning them
• already completed reindex steps are detected, see comment https://github.com/camunda/camunda-optimize/pull/2929#discussion_r529737353