Query MX records during provider autoconfiguration

Previously MX records were queried only for OAuth 2 configuration and
did not affect the list of servers tried. User was required to manually
configure the servers for Google Workspace (former GSuite) domains.

Now MX records are queried during configuration. If provider is found in
offline database, its ID, corresponding to the filename, is saved as
`configured_provider`.

`configured_provider` is also set during database migration if email
address uses the domain from the provider database, but no MX querying
is done.

examples/repl/cmdline.rs

@@ -1055,7 +1055,7 @@ pub async fn cmdline(context: Context, line: &str, chat_id: &mut ChatId) -> Resu
         }
         "providerinfo" => {
             ensure!(!arg1.is_empty(), "Argument <addr> missing.");
-            match provider::get_provider_info(arg1) {
+            match provider::get_provider_info(arg1).await {
                 Some(info) => {
                     println!("Information for provider belonging to {}:", arg1);
                     println!("status: {}", info.status as u32);