Tidy up includes and add modules to the classic build:

An alternative to the unity build, the classic build compiles each
translation unit individually. This adds more modules to the classic build:

* Remove unity header app.h
* Add missing includes as needed
* Remove obsolete NodeStore backend code
* Add app/, core/, crypto/, json/, net/, overlay/, peerfinder/ to classic build

src/ripple/app/tx/TransactionEngine.cpp

@@ -17,6 +17,15 @@
 */
 //==============================================================================
 
+#include <BeastConfig.h>
+#include <ripple/app/tx/TransactionEngine.h>
+#include <ripple/app/transactors/Transactor.h>
+#include <ripple/basics/Log.h>
+#include <ripple/json/to_string.h>
+#include <ripple/protocol/Indexes.h>
+#include <boost/foreach.hpp>
+#include <cassert>
+
 namespace ripple {
 
 //