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/imap/idle.rs

@@ -25,7 +25,7 @@ impl Imap {
         if !self.can_idle() {
             bail!("IMAP server does not have IDLE capability");
         }
-        self.setup_handle(context).await?;
+        self.prepare(context).await?;
 
         self.select_folder(context, watch_folder.as_deref()).await?;
 
@@ -158,7 +158,7 @@ impl Imap {
                     // try to connect with proper login params
                     // (setup_handle_if_needed might not know about them if we
                     // never successfully connected)
-                    if let Err(err) = self.connect_configured(context).await {
+                    if let Err(err) = self.prepare(context).await {
                         warn!(context, "fake_idle: could not connect: {}", err);
                         continue;
                     }