Add fsync-store-paths option

- Add recursiveSync function to flush a directory tree to disk

- Add AutoCloseFD::startFsync to initiate an asynchronous fsync
  without waiting for the result

- Initiate an asynchronous fsync while extracting NAR files

- Implement the fsync-store-paths option in LocalStore

src/libstore/globals.hh

@@ -234,6 +234,13 @@ public:
           default is `true`.
         )"};
 
+    Setting<bool> fsyncStorePaths{this, false, "fsync-store-paths",
+        R"(
+        "Whether to call `fsync()` on store paths before registering them, to
+         flush them to disk. This improves robustness in case of system crashes,
+         but reduces performance. The default is `false`.
+        )"};
+
     Setting<bool> useSQLiteWAL{this, !isWSL1(), "use-sqlite-wal",
         "Whether SQLite should use WAL mode."};