background-sync: spread node updates over time.

[marseel]

Before, depending on cluster-size we were triggering node update for each node at fixed intervals depending on cluster-size. This resulted in high cpu usage spike in agent. While the intent is to fix state that got stale and shouldn't be the primary source of updates, it makes sense to spread these updates over time to average out cpu usage.

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Fixes: #issue-number

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[marseel]

/test-backport-1.13