Harden validations:

This commit introduces the "HardenedValidations" amendment which,
if enabled, allows validators to include additional information in
their validations that can increase the robustness of consensus.

Specifically, the commit introduces a new optional field that can
be set in validation messages can be used to attest to the hash of
the latest ledger that a validator considers to be fully validated.

Additionally, the commit leverages the previously introduced "cookie"
field to improve the robustness of the network by making it possible
for servers to automatically detect accidental misconfiguration which
results in two or more validators using the same validation key.

src/ripple/protocol/Feature.h

@@ -111,7 +111,7 @@ class FeatureCollections
         "RequireFullyCanonicalSig",
         "fix1781",  // XRPEndpointSteps should be included in the circular
                     // payment check
-    };
+        "HardenedValidations"};
 
     std::vector<uint256> features;
     boost::container::flat_map<uint256, std::size_t> featureToIndex;
@@ -366,6 +366,7 @@ extern uint256 const featureDeletableAccounts;
 extern uint256 const fixQualityUpperBound;
 extern uint256 const featureRequireFullyCanonicalSig;
 extern uint256 const fix1781;
+extern uint256 const featureHardenedValidations;
 
 // The following amendments have been active for at least two years.
 // Their pre-amendment code has been removed.