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.h

@@ -6,7 +6,10 @@
 #define BITCOIN_KEYSTORE_H
 
 #include "crypter.h"
-#include "script.h"
+#include "util.h"
+#include "base58.h"
+
+class CScript;
 
 /** A virtual base class for key stores */
 class CKeyStore