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/libutil/archive.hh

@@ -95,7 +95,7 @@ struct RetrieveRegularNARSink : ParseSink
 
 void parseDump(ParseSink & sink, Source & source);
 
-void restorePath(const Path & path, Source & source);
+void restorePath(const Path & path, Source & source, bool startFsync = false);
 
 /* Read a NAR from 'source' and write it to 'sink'. */
 void copyNAR(Source & source, Sink & sink);