Cache clear is not working if the site uses HTTP Auth

Under the hood, drupal_http_request() is used to send the requests to clear the site cache. If drupal_http_request() has an $options value that helps with that, it can be done. Otherwise, the alternative is allowing those requests that come from the same server.

It is possible to pass credentials to drupal_http_request(). Since the credentials should be written on the settings.php file, they should not be written as plain text.

The Drush support code has been removed. It could eventually be re-added, but it will be implemented differently.