registry: Replace reward queue with a deterministic address scheme

[armaniferrante]

Instead of using an event queue for clients to keep track of available rewards, we can use deterministic addresses. Specifically, we can

• Add a global reward vendor counter to the Registrar account
• Use program derived addresses (owned by the program) for each reward vendor, using seeds as `[registrar.key.as_ref(), &"reward".as_le_bytes(), &[bump]].
• Pass in the next reward available on stake and unstake to perform reward eligibility checks (instead of passing in the reward queue account as is done now)