Add the ability to configure caching the homepage

[tampakis]

Add support to cache the solr query response for loading the facet counts on the homepage with an option to set how often the cached response needs to be updated.

[cbeer]

There isn't a great seam for doing this; the best we came up with in one of our apps was:

https://github.com/sul-dlss/dlme/blob/master/app/controllers/spotlight/home_pages_controller.rb#L32-L37

Maybe some work in Blacklight::Solr::Repository could push caching down there?