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/scan_folders.rs

@@ -25,7 +25,7 @@ impl Imap {
         }
         info!(context, "Starting full folder scan");
 
-        self.connect_configured(context).await?;
+        self.prepare(context).await?;
         let session = self.session.as_mut();
         let session = session.context("scan_folders(): IMAP No Connection established")?;
         let folders: Vec<_> = session.list(Some(""), Some("*")).await?.collect().await;