Add manual cache clear, somewhat refined

Since #1515372: Caching Results there is a (very) persistent cache. That's good, because a street's location on the planet doesn't change very often. Yet, you might want to redo all or some queries, because something else has changed.

Since empty results are cached too (by default), it'd be good to separate the cache flush: all caches, only no-results caches.

And maybe a cache summary, to give an admin some perspective.