Consolidate transaction signature checking.

* All checks flow through ripple::checkValidity, which transparently caches result flags.
* All external transaction submission code paths use checkValidity.
* SF_SIGGOOD flag no longer appears outside of HashRouter / checkValidity.
* Validity can be forced in known or trusted scenarios.

src/ripple/app/tx/impl/SetSignerList.cpp

@@ -75,8 +75,8 @@ TER
 SetSignerList::preflight (PreflightContext const& ctx)
 {
     if (! (ctx.flags & tapENABLE_TESTING) &&
-        ! ctx.rules.enabled(featureMultiSign,
-            ctx.config.features))
+            ! ctx.rules.enabled(featureMultiSign,
+                ctx.app.config().features))
         return temDISABLED;
 
     auto const ret = preflight1 (ctx);