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/key.h

@@ -12,7 +12,7 @@
 #include <openssl/ecdsa.h>
 #include <openssl/obj_mac.h>
 
-#include "serialize.h"
+// #include "serialize.h"
 #include "uint256.h"
 
 // secp160k1