Move path info caching from BinaryCacheStore to Store

Caching path info is generally useful. For instance, it speeds up "nix
path-info -rS /run/current-system" (i.e. showing the closure sizes of
all paths in the closure of the current system) from 5.6s to 0.15s.

This also eliminates some APIs like Store::queryDeriver() and
Store::queryReferences().

src/libutil/lru-cache.hh

@@ -79,6 +79,12 @@ public:
     {
         return data.size();
     }
+
+    void clear()
+    {
+        data.clear();
+        lru.clear();
+    }
 };
 
 }