imap: refactor to always create Imap configured

`Imap` structure is always created in a configured state now. There is
no default value for `ImapConfig` anymore.

Also resultify Scheduler::start() to fail on database errors, for
example if IMAP configuration cannot be read from the database during
`start_io()`. Previosuly errors during reading keys such as
`mvbox_watch` were simply ignored and folders were not watched until
the application is completely restarted, now start_io() will fail and
scheduler will only be started at the next start_io() call which
usually happens when app is brought to the foreground.

src/context.rs

@@ -161,7 +161,9 @@ impl Context {
 
         {
             let l = &mut *self.inner.scheduler.write().await;
-            l.start(self.clone()).await;
+            if let Err(err) = l.start(self.clone()).await {
+                error!(self, "Failed to start IO: {}", err)
+            }
         }
     }