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/test/transaction_tests.cpp

@@ -11,7 +11,7 @@
 #include "core_io.h"
 #include "key.h"
 #include "keystore.h"
-#include "main.h"
+#include "main.h" // For CheckTransaction
 #include "policy/policy.h"
 #include "script/script.h"
 #include "script/script_error.h"