Safely recover connected pages after deployment

[leandrocp]

Users get a "Not Found" error message after a page reconnects, while they should see the updated content.

Steps to reproduce:
1 - Open a page and keep it open
2 - Deploy the site

The page should reconnect gracefully and display the updated content (some configuration may have changed after the new deployment), but instead they see a "Not Found" error message.

That happens because the loading process is async and the moment LiveView reconnects and tries to render the page process, that page may have not been loaded yet (not ready) so it fallbacks to displaying an error page.

Making the loading process sync brings some issues, the main one is demanding too much resources and crashing the BEAM, so we need to find a way to recover such connected pages but not all of them.

Related to #400

[leandrocp]

Closed by #434