reduce running goroutines for volumewatcher

[tgross]

In #7794, @langmartin pointed out:

For the long term, I'm a little worried about starting a go routine for every volume. It seems at least possible that some operators could set us up for more goroutines than we want running for these, maybe in a followup if warranted we could use a fixed pool?

For large clusters with many volumes we could end up with a lot of extra running goroutines. We could reduce this with a fixed pool. But we may also be able to simply Stop() the watch goroutine when we've completed processing an incoming update, when we've determined either there are no more past claims to process or no more claims at all).

[github-actions]

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