Use tokio pool and spawn instead of a Spotify server on separate thread and a proxy object

[kstep]

• [ ] Pass around tokio::runtime::Handle and Arc<RwLock<Spotify>> instead of SpotifyProxy.
• [ ] Use Handle::spawn() and direct async calls to Spotify methods instead of message passing.
• [ ] Remove SpotifyProxy and SpotifyServer.
• [ ] Maybe join up Handle and Spotify structs into a single struct and add convenience wrappers for async methods.