Remove blocking code and spawn_blocking

Rayhunter uses a mixture of spawn and spawn_blocking, then also does
some blocking operations inside of async code.

Move everything to async. This allows us to use the single-threaded
runtime.

Now the binary is 100kB smaller, and the memory usage also improved by
~100kB on tplink.

daemon/src/main.rs

@@ -182,7 +182,7 @@ fn run_shutdown_thread(
     })
 }
 
-#[tokio::main]
+#[tokio::main(flavor = "current_thread")]
 async fn main() -> Result<(), RayhunterError> {
     env_logger::init();