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/mempool_tests.cpp

@@ -2,7 +2,6 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include "main.h"
 #include "txmempool.h"
 #include "util.h"