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/script.cpp

@@ -1092,7 +1092,7 @@ uint256 SignatureHash(CScript scriptCode, const CTransaction& txTo, unsigned int
     }
 
     // Serialize and hash
-    CDataStream ss(SER_GETHASH);
+    CDataStream ss(SER_GETHASH, 0);
     ss.reserve(10000);
     ss << txTmp << nHashType;
     return Hash(ss.begin(), ss.end());