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/init.cpp

@@ -7,7 +7,6 @@
 #include "bitcoinrpc.h"
 #include "net.h"
 #include "init.h"
-#include "strlcpy.h"
 #include "util.h"
 #include "ui_interface.h"
 #include <boost/filesystem.hpp>