Refactor Feature name management and creation:

* Only require adding the new feature names in one place. (Also need to
  increment a counter, but a check on startup will catch that.)
* Allows rippled to have the code to support a given amendment, but
  not vote for it by default. This allows the amendment to be enabled in
  a future version without necessarily amendment blocking these older
  versions.
* The default vote is carried with the amendment name in the list of
  supported amendments.
* The amendment table is constructed with the amendment and default
  vote.

src/test/jtx/Env_test.cpp

@@ -812,6 +812,7 @@ public:
 
         auto const missingSomeFeatures =
             supported_amendments() - featureMultiSignReserve - featureFlow;
+        BEAST_EXPECT(missingSomeFeatures.count() == (supported.count() - 2));
         {
             // a Env supported_features_except is missing *only* those features
             Env env{*this, missingSomeFeatures};