Separate BackendFetcher from HealthCheck

[djshow832]

Development Task

Currently, BackendFetcher fetches the backend list every 3 seconds, together with health check. When fetching backends fails, it infinitely retries.

However, TiProxy doesn't need to fetch the backends so frequently, especially when there are tens of TiProxy and PD becomes the bottleneck, TiProxy may take more time to fetch the backend list, delaying health check. For scale-in, TiProxy can detect it from health check. For scale-out, TiProxy doesn't need to detect it so in time.