Run flush commands with FlushMode on upstreams only

[schnapster]

Ahoy, some of the flush commands are executed on all nodes of a cluster, but should only run on upstreams. This PR adds tests and code changes to address that. I noticed this while working on a solution for spring-projects/spring-data-redis#2187

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[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.