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/wallet/walletdb.cpp

@@ -7,7 +7,7 @@
 
 #include "base58.h"
 #include "consensus/validation.h"
-#include "main.h"
+#include "main.h" // For CheckTransaction
 #include "protocol.h"
 #include "serialize.h"
 #include "sync.h"