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/keystore.cpp

@@ -4,7 +4,6 @@
 // file license.txt or http://www.opensource.org/licenses/mit-license.php.
 
 #include "keystore.h"
-#include "crypter.h"
 #include "script.h"
 
 bool CKeyStore::GetPubKey(const CBitcoinAddress &address, std::vector<unsigned char> &vchPubKeyOut) const