Improve amendment processing and activation logic:

* The amendment ballot counting code contained a minor technical
  flaw, caused by the use of integer arithmetic and rounding
  semantics, that could allow amendments to reach majority with
  slightly less than 80% support. This commit introduces an
  amendment which, if enabled, will ensure that activation
  requires at least 80% support.
* This commit also introduces a configuration option to adjust
  the amendment activation hysteresis. This option is useful on
  test networks, but should not be used on the main network as
  is a network-wide consensus parameter that should not be
  changed on a per-server basis; doing so can result in a
  hard-fork.

Fixes #3396

src/ripple/protocol/impl/Feature.cpp

@@ -130,7 +130,8 @@ detail::supportedAmendments()
         "fixQualityUpperBound",
         "RequireFullyCanonicalSig",
         "fix1781",
-        "HardenedValidations"};
+        "HardenedValidations",
+        "fixAmendmentMajorityCalc"};
     return supported;
 }
 
@@ -181,7 +182,8 @@ uint256 const
     fixQualityUpperBound            = *getRegisteredFeature("fixQualityUpperBound"),
     featureRequireFullyCanonicalSig = *getRegisteredFeature("RequireFullyCanonicalSig"),
     fix1781                         = *getRegisteredFeature("fix1781"),
-    featureHardenedValidations      = *getRegisteredFeature("HardenedValidations");
+    featureHardenedValidations      = *getRegisteredFeature("HardenedValidations"),
+    fixAmendmentMajorityCalc        = *getRegisteredFeature("fixAmendmentMajorityCalc");
 
 // The following amendments have been active for at least two years. Their
 // pre-amendment code has been removed and the identifiers are deprecated.