nix-daemon: Disable path info cache

This is useless because the client also caches path info, and can
cause problems for long-running clients like hydra-queue-runner
(i.e. it may return cached info about paths that have been
garbage-collected).

src/libstore/store-api.hh

@@ -241,7 +241,7 @@ protected:
 
     struct State
     {
-        LRUCache<std::string, std::shared_ptr<ValidPathInfo>> pathInfoCache{64 * 1024};
+        LRUCache<std::string, std::shared_ptr<ValidPathInfo>> pathInfoCache;
     };
 
     Sync<State> state;
@@ -252,6 +252,11 @@ protected:
 
 public:
 
+    size_t getCacheSize()
+    {
+        return state.lock()->pathInfoCache.size();
+    }
+
     virtual ~Store() { }
 
     virtual std::string getUri() = 0;