Use requestAsync/postAsync for purging

This week there was a problem reaching fastly (Jun 28, 2017).

When fastly is hard to reach, or not reachable at all, this results in an (sometimes extremely) degraded performance hit for sites when trying to purge keys, especially when the site is very busy and a lot of invalidations are happening. Because the request() call is waiting for a response, the webserver isn't unable to handle other requests and everything simply starts piling up very fast until the very moment the site can become completely response for authenticated users.

The proposal would be to use requestAsync() from the http client because the rational is simple:

- if the fastly is up, then we still want the client to have a quick response without it having to know that we are invalidating stuff on the server for anonymous users
- if fastly is down, then the authenticated user should still be able to perform requests, invalidations within drupal will still happen (e.g. render cache etc). Since fastly can't be reached during these times, anonymous pages won't yet be invalidated, but we can do this either manually ourselves later, or the page which couldn't be invalidated yet, will be on a later time.

Using async means we won't be able to detect whether the response will be ok or not, but that's ok if development testing has shown that the initial setup with keys went ok.