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/checkblock_tests.cpp

@@ -4,7 +4,7 @@
 
 #include "clientversion.h"
 #include "consensus/validation.h"
-#include "main.h"
+#include "main.h" // For CheckBlock
 #include "primitives/block.h"
 #include "test/test_bitcoin.h"
 #include "utiltime.h"