chore: Fix unbounded channel

For the refresh JWKS operation, a worker fiber was reading from a notification channel that had no buffering.

This has two issues. The first issue has to do with memory growth, but the second is more subtle. When we don't supply any bounds, the code needs to be capable to handle concurrent requests and the worker must be able to dedup notifications. While this is what happens now, a future iteration might introduce a bugthat removes the dedup logic.