Begin factoring out the protocol code

Use `nix::serve_proto::queryValidPaths`.

src/hydra-queue-runner/build-remote.cc

@@ -117,13 +117,11 @@ static void copyClosureTo(std::timed_mutex & sendMutex, Store & destStore,
        garbage-collect paths that are already there. Optionally, ask
        the remote host to substitute missing paths. */
     // FIXME: substitute output pollutes our build log
-    conn.to << cmdQueryValidPaths << 1 << useSubstitutes;
-    worker_proto::write(destStore, conn.to, closure);
-    conn.to.flush();
-
     /* Get back the set of paths that are already valid on the remote
        host. */
-    auto present = worker_proto::read(destStore, conn.from, Phantom<StorePathSet> {});
+    auto present = serve_proto::queryValidPaths(
+        conn, destStore,
+        true, closure, useSubstitutes);
 
     if (present.size() == closure.size()) return;