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/pow_tests.cpp

@@ -4,7 +4,6 @@
 
 #include "chain.h"
 #include "chainparams.h"
-#include "main.h"
 #include "pow.h"
 #include "random.h"
 #include "util.h"