Close rate-limiter when closing pool

That rate-limiter instances are never closed, which lead to thousands of stale goroutines in my case. Because `singleConnPool` is not public, https://github.com/bsm/redis-balancer/blob/master/backend.go#L83 is creating normal client connections for its background checks. When the client is closed, the pool is released too, but the `rateLimiter.loop` is never shut down properly.