Further reduce header dependencies

This commit removes the dependency of serialize.h on PROTOCOL_VERSION,
and makes this parameter required instead of implicit. This is much saner,
as it makes the places where changing a version number can have an
influence obvious.

src/wallet.h

@@ -6,7 +6,6 @@
 #define BITCOIN_WALLET_H
 
 #include "main.h"
-#include "bignum.h"
 #include "key.h"
 #include "keystore.h"
 #include "script.h"