Break dependency of init on wallet.

This required some code movement (what was CWalletTx::AcceptToMemoryPool
doing in main?), and adding a few explicit includes that used to be
implicit through init.h.

src/init.h

@@ -5,7 +5,10 @@
 #ifndef BITCOIN_INIT_H
 #define BITCOIN_INIT_H
 
-#include "wallet.h"
+#include <string>
+#include <boost/thread.hpp>
+
+class CWallet;
 
 extern std::string strWalletFile;
 extern CWallet* pwalletMain;