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/app/main/Application.cpp

@@ -77,9 +77,6 @@
 
 namespace ripple {
 
-// 204/256 about 80%
-static int const MAJORITY_FRACTION(204);
-
 //------------------------------------------------------------------------------
 
 namespace detail {
@@ -1533,8 +1530,7 @@ ApplicationImp::setup()
         Section enabledAmendments = config_->section(SECTION_AMENDMENTS);
 
         m_amendmentTable = make_AmendmentTable(
-            weeks{2},
-            MAJORITY_FRACTION,
+            config().AMENDMENT_MAJORITY_TIME,
             supportedAmendments,
             enabledAmendments,
             config_->section(SECTION_VETO_AMENDMENTS),