Register `Refresh()` as a scheduled service

[JeremyCaney]

With the wire-up of services migrated to .NET's built-in DI container (#137), we can move the periodic cache-refresh job out of the request path (where it delays intervalled requests) and into a proper background service that runs at startup.

• Add a hosted service that warms the cache at startup
  • Must hook StartingAsync, not StartAsync, or the server can start accepting requests before the cache is warm
• Runs the periodic Refresh() in the background (new TopicCacheRefreshService : IHostedLifecycleService`)