Get rid of VARINT default argument

This removes the need for the GNU C++ extension of variadic macros.

src/txdb.cpp

@@ -339,7 +339,7 @@ public:
                 ::Unserialize(s, Using<TxOutCompression>(vout[i]));
         }
         // coinbase height
-        ::Unserialize(s, VARINT(nHeight, VarIntMode::NONNEGATIVE_SIGNED));
+        ::Unserialize(s, VARINT_MODE(nHeight, VarIntMode::NONNEGATIVE_SIGNED));
     }
 };