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/Feature.h

@@ -70,8 +70,7 @@ namespace detail {
 class FeatureCollections
 {
     static constexpr char const* const featureNames[] = {
-        "MultiSign",  // Unconditionally supported.
-        "Tickets",
+        "MultiSign",      // Unconditionally supported.
         "TrustSetAuth",   // Unconditionally supported.
         "FeeEscalation",  // Unconditionally supported.
         "OwnerPaysFee",
@@ -113,7 +112,9 @@ class FeatureCollections
                     // payment check
         "HardenedValidations",
         "fixAmendmentMajorityCalc",  // Fix Amendment majority calculation
-        "NegativeUNL"};
+        "NegativeUNL",
+        "TicketBatch"};
+
     std::vector<uint256> features;
     boost::container::flat_map<uint256, std::size_t> featureToIndex;
     boost::container::flat_map<std::string, uint256> nameToFeature;
@@ -343,7 +344,6 @@ foreachFeature(FeatureBitset bs, F&& f)
             f(bitsetIndexToFeature(i));
 }
 
-extern uint256 const featureTickets;
 extern uint256 const featureOwnerPaysFee;
 extern uint256 const featureFlow;
 extern uint256 const featureCompareTakerFlowCross;
@@ -370,6 +370,7 @@ extern uint256 const fix1781;
 extern uint256 const featureHardenedValidations;
 extern uint256 const fixAmendmentMajorityCalc;
 extern uint256 const featureNegativeUNL;
+extern uint256 const featureTicketBatch;
 
 }  // namespace ripple