bbolt-cache: debug log when cleaning objects, and throttle to once per 1s

[sttts]

We are continously cleaning objects it seems. In upbound/sttts-2000-claims roughly 25-30 per second, despite no requests.

This PR makes that visible.

Test locally by running with the playground again upper controlplane.

[avalanche123]

we'd want to stop the process here normally.

[sttts]

fixed.

For context, it's printed here and the cache is removed:

	go func() {
		err := ca.Start(ctx)
		log.Debug("Cache stopped", "error", err)

		// Start blocks until ctx is closed, or it encounters an error. If we make
		// it here either the cache crashed, or the context was cancelled (e.g.
		// because our session expired).
		c.remove(id)
	}()

So, it's not the process in the unix sense that is stopped, but the cache.