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/qt/messagepage.cpp

@@ -91,7 +91,7 @@ void MessagePage::on_signMessage_clicked()
         return;
     }
 
-    CDataStream ss(SER_GETHASH);
+    CDataStream ss(SER_GETHASH, 0);
     ss << strMessageMagic;
     ss << ui->message->document()->toPlainText().toStdString();