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/nodestore/backend/NullFactory.cpp

@@ -17,6 +17,7 @@
 */
 //==============================================================================
 
+#include <BeastConfig.h>
 #include <ripple/nodestore/Factory.h>
 #include <ripple/nodestore/Manager.h>
 #include <beast/cxx14/memory.h> // <memory>