Add STAmount switchover support to tx queue:

* RIPD-1513
* New fix1513 amendment

src/test/rpc/LedgerRPC_test.cpp

@@ -477,7 +477,7 @@ class LedgerRPC_test : public beast::unit_test::suite
                     .set("minimum_txn_in_ledger_standalone", "3");
                 return cfg;
             }),
-            with_features(featureFeeEscalation)};
+            with_features(featureFeeEscalation, fix1513)};
 
         Json::Value jv;
         jv[jss::ledger_index] = "current";