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/ripple/protocol/impl/ErrorCodes.cpp

@@ -39,6 +39,7 @@ constexpr static ErrorInfo unorderedErrorInfos[]{
     {rpcAMENDMENT_BLOCKED,
      "amendmentBlocked",
      "Amendment blocked, need upgrade."},
+    {rpcEXPIRED_VALIDATOR_LIST, "unlBlocked", "Validator list expired."},
     {rpcATX_DEPRECATED,
      "deprecated",
      "Use the new API or specify a ledger range."},