Change features default behavior in Env (RIPD-1460):

Enable all supported amendments in Env by default. Rename `features()`
to `with_features()` and add `all_features_except()` to support feature
subsets in Env. Refactor internal feature handling based on a bitset.

src/ripple/core/impl/Config.cpp

@@ -534,7 +534,13 @@ void Config::loadFromString (std::string const& fileContents)
     {
         auto const part = section("features");
         for(auto const& s : part.values())
-            features.insert(feature(s));
+        {
+            if (auto const f = getRegisteredFeature(s))
+                features.insert(*f);
+            else
+                Throw<std::runtime_error>(
+                    "Unknown feature: " + s + "  in config file.");
+        }
     }
 }