Allow facet to be cached

Thanks @prudloff, this would be a great addition. I wanted to review the patch, but ran into problems, and not sure of the best method to verify a cache hit ... Should I just check to see if these browser headers change?

{
  "name": "x-drupal-cache",
  "value": "UNCACHEABLE (response policy)"
},
{
  "name": "x-drupal-dynamic-cache",
  "value": "UNCACHEABLE (poor cacheability)"
},

Yes looking at those headers is helpful but it can be tricky to test if you have other parts of the page that prevent caching.

Another way to test this is to save your view. Without the patch, the view display will have this config:

    cache_metadata:
      max-age: 0

With the patch, it should turn to -1 (if your view is cacheable of course).