Support UNLs with future effective dates:

* Creates a version 2 of the UNL file format allowing publishers to
  pre-publish the next UNL while the current one is still valid.
* Version 1 of the UNL file format is still valid and backward
  compatible.
* Also causes rippled to lock down if it has no valid UNLs, similar to
  being amendment blocked, except reversible.
* Resolves #3548
* Resolves #3470

src/test/net/DatabaseDownloader_test.cpp

@@ -40,6 +40,8 @@ class DatabaseDownloader_test : public beast::unit_test::suite
             env.app().getIOService(),
             list,
             env.timeKeeper().now() + std::chrono::seconds{3600},
+            // No future VLs
+            {},
             ssl);
     }