Feature request: revalidate some stale caches in the background

[Cellard]

It would be nice to have an option to refresh some stale caches in the background. With such mechanism, middleware could increment the overall application performance.

At least stale-while-revalidate directive explicitly says about revalidating of responses in the background.

Beyond that, we could invent an explicit way to manage refreshable caches. For example:

Make a request:

// Possible way to mark a request as refreshable — add 'refreshable' to its config
$client = new Client($config + ['refreshable' => true]);
$client->get('https://example.com/resource');

Then, run as a scheduled job:

// Possible way to select requests for refreshing — filter caches with method/url
$magic->revalidateStaled(['https://example.com/']);

If @Kevinrob says this is possible and reasonble — I could try to make a PR?