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Fix unnecessary too-old-errors from watch cache

When initializing watch cache via LIST, we set its resource version to the RV of the list request.
However, before this PR, the first incoming watch event (updating the watch cache) was moving the "smallest oldest known version" to RV of that watch event. So watch requests passing rv equal to the RV returned from the initial list were failing with "too old resource version".
That is not needed, because we know that in the meantime there weren't any other watch events.

This PR is addressing that issue.

/assign @liggitt