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/ErrorCodes.h

@@ -134,8 +134,9 @@ enum error_code_i {
     rpcDB_DESERIALIZATION = 77,
     rpcEXCESSIVE_LGR_RANGE = 78,
     rpcINVALID_LGR_RANGE = 79,
+    rpcEXPIRED_VALIDATOR_LIST = 80,
     rpcLAST =
-        rpcINVALID_LGR_RANGE  // rpcLAST should always equal the last code.=
+        rpcEXPIRED_VALIDATOR_LIST  // rpcLAST should always equal the last code.
 };
 
 /** Codes returned in the `warnings` array of certain RPC commands.
@@ -145,6 +146,7 @@ enum error_code_i {
 enum warning_code_i {
     warnRPC_UNSUPPORTED_MAJORITY = 1001,
     warnRPC_AMENDMENT_BLOCKED = 1002,
+    warnRPC_EXPIRED_VALIDATOR_LIST = 1003,
 };
 
 //------------------------------------------------------------------------------

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."},

src/ripple/protocol/jss.h

@@ -143,6 +143,8 @@ JSS(bids);                   // out: Subscribe
 JSS(binary);                 // in: AccountTX, LedgerEntry,
                              //     AccountTxOld, Tx LedgerData
 JSS(blob);                   // out: ValidatorList
+JSS(blobs_v2);               // out: ValidatorList
+                             // in: UNL
 JSS(books);                  // in: Subscribe, Unsubscribe
 JSS(both);                   // in: Subscribe, Unsubscribe
 JSS(both_sides);             // in: Subscribe, Unsubscribe
@@ -211,6 +213,8 @@ JSS(directory);               // in: LedgerEntry
 JSS(domain);                  // out: ValidatorInfo, Manifest
 JSS(drops);                   // out: TxQ
 JSS(duration_us);             // out: NetworkOPs
+JSS(effective);               // out: ValidatorList
+                              // in: UNL
 JSS(enabled);                 // out: AmendmentTable
 JSS(engine_result);           // out: NetworkOPs, TransactionSign, Submit
 JSS(engine_result_code);      // out: NetworkOPs, TransactionSign, Submit
@@ -439,8 +443,10 @@ JSS(random);                // out: Random
 JSS(raw_meta);              // out: AcceptedLedgerTx
 JSS(receive_currencies);    // out: AccountCurrencies
 JSS(reference_level);       // out: TxQ
+JSS(refresh_interval);      // in: UNL
 JSS(refresh_interval_min);  // out: ValidatorSites
 JSS(regular_seed);          // in/out: LedgerEntry
+JSS(remaining);             // out: ValidatorList
 JSS(remote);                // out: Logic.h
 JSS(request);               // RPC
 JSS(requested);             // out: Manifest
@@ -471,6 +477,7 @@ JSS(seq);                       // in: LedgerEntry;
                                 // out: NetworkOPs, RPCSub, AccountOffers,
                                 //      ValidatorList, ValidatorInfo, Manifest
 JSS(seqNum);                    // out: LedgerToJson
+JSS(sequence);                  // in: UNL
 JSS(sequence_count);            // out: AccountInfo
 JSS(server_domain);             // out: NetworkOPs
 JSS(server_state);              // out: NetworkOPs