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/skiplist_tests.cpp

@@ -3,7 +3,6 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include "chain.h"
-#include "main.h"
 #include "random.h"
 #include "util.h"
 #include "test/test_bitcoin.h"