refactor: Un-callback transform Store::queryPathInfoFromClientCache

This part of the code was not necessarily callback based.
Removing CPS is always nice; particularly if there's no loss of
functionality, like here.

src/libstore/store-api.hh

@@ -285,14 +285,14 @@ public:
     /**
      * NOTE: this is not the final interface - to be modified in next commit.
      *
-     * Asynchronous version that only queries the local narinfo cache and not
+     * Version of queryPathInfo() that only queries the local narinfo cache and not
      * the actual store.
      *
-     * @return true if the path was known and the callback invoked
-     * @return false if the path was not known and the callback not invoked
+     * @return `std::make_optional(vpi)` if the path is known
+     * @return `std::null_opt` if the path was not known to be valid or invalid
      * @throw InvalidPathError if the path is known to be invalid
      */
-    bool queryPathInfoFromClientCache(const StorePath & path, Callback<ref<const ValidPathInfo>> & callback);
+    std::optional<ref<const ValidPathInfo>> queryPathInfoFromClientCache(const StorePath & path);
 
     /**
      * Query the information about a realisation.