fileutils: move API to std::filesystem, cleanup

Change-Id: I5408d193bda6830395bd705371c86c949643ee74
diff --git a/include/turn_cache.h b/include/turn_cache.h
index deeb919..a971e7f 100644
--- a/include/turn_cache.h
+++ b/include/turn_cache.h
@@ -28,6 +28,7 @@
 #include <mutex>
 #include <optional>
 #include <string>
+#include <filesystem>
 
 namespace dht {
 namespace log {
@@ -65,7 +66,7 @@
 
 private:
     std::string accountId_;
-    std::string cachePath_;
+    std::filesystem::path cachePath_;
     TurnTransportParams params_;
     std::atomic_bool enabled_ {false};
     /**