Includes: Cleanup around net main and wallet

-Move from .h to .cpp: in main, net and wallet
-Remove unnecessary #include "main.h"
-Cleanup some wallet files includes

src/qt/optionsdialog.cpp

@@ -13,7 +13,7 @@
 #include "guiutil.h"
 #include "optionsmodel.h"
 
-#include "main.h" // for MAX_SCRIPTCHECK_THREADS
+#include "main.h" // for DEFAULT_SCRIPTCHECK_THREADS and MAX_SCRIPTCHECK_THREADS
 #include "netbase.h"
 #include "txdb.h" // for -dbcache defaults