Implement enhanced Ticket support:

Tickets are a mechanism to allow for the "out-of-order" execution of
transactions on the XRP Ledger.

This commit, if merged, reworks the existing support for tickets and
introduces support for 'ticket batching', completing the feature set
needed for tickets.

The code is gated under the newly-introduced `TicketBatch` amendment
and the `Tickets` amendment, which is not presently active on the
network, is being removed.

The specification for this change can be found at:
https://github.com/xrp-community/standards-drafts/issues/16

src/ripple/protocol/impl/Feature.cpp

@@ -92,7 +92,6 @@ detail::supportedAmendments()
     // Removing them will cause servers to become amendment blocked.
     static std::vector<std::string> const supported{
         "MultiSign",      // Unconditionally supported.
-                          //        "Tickets",
         "TrustSetAuth",   // Unconditionally supported.
         "FeeEscalation",  // Unconditionally supported.
                           //        "OwnerPaysFee",
@@ -132,7 +131,8 @@ detail::supportedAmendments()
         "fix1781",
         "HardenedValidations",
         "fixAmendmentMajorityCalc",
-        //"NegativeUNL"      // Commented out to prevent automatic enablement
+        //"NegativeUNL",      // Commented out to prevent automatic enablement
+        //"TicketBatch",      // Commented out to prevent automatic enablement
     };
     return supported;
 }
@@ -160,7 +160,6 @@ bitsetIndexToFeature(size_t i)
 // clang-format off
 
 uint256 const
-    featureTickets                  = *getRegisteredFeature("Tickets"),
     featureOwnerPaysFee             = *getRegisteredFeature("OwnerPaysFee"),
     featureFlow                     = *getRegisteredFeature("Flow"),
     featureCompareTakerFlowCross    = *getRegisteredFeature("CompareTakerFlowCross"),
@@ -186,7 +185,8 @@ uint256 const
     fix1781                         = *getRegisteredFeature("fix1781"),
     featureHardenedValidations      = *getRegisteredFeature("HardenedValidations"),
     fixAmendmentMajorityCalc        = *getRegisteredFeature("fixAmendmentMajorityCalc"),
-    featureNegativeUNL              = *getRegisteredFeature("NegativeUNL");
+    featureNegativeUNL              = *getRegisteredFeature("NegativeUNL"),
+    featureTicketBatch              = *getRegisteredFeature("TicketBatch");
 
 // The following amendments have been active for at least two years. Their
 // pre-amendment code has been removed and the identifiers are deprecated.