scheduler: make Scheduler stateless

Scheduler has no Stopped state anymore. If Scheduler exists, it is
always started. Scheduler is stopped via Scheduler.stop(), which
consumes Scheduler and cannot fail.

src/configure.rs

@@ -60,7 +60,7 @@ impl Context {
         use futures::future::FutureExt;
 
         ensure!(
-            !self.scheduler.read().await.is_running(),
+            self.scheduler.read().await.is_none(),
             "cannot configure, already running"
         );
         ensure!(