public boolean isPluginDownloaded(String url) {
        String pluginFilename = url.substring(url.lastIndexOf("/") + 1);
        return XMPPServer.getInstance().getPluginManager().isInstalled( pluginFilename);
    }
Returns true if the plugin downloaded from the specified URL has been downloaded. Plugins
may be downloaded but not installed. The install process may take like 30 seconds to
detect new plugins to install.

@param url the URL of the latest version of the plugin.
@return true if the plugin downloaded from the specified URL has been downloaded.
xmppserver/src/main/java/org/jivesoftware/openfire/update/UpdateManager.java