src: don't call v8::Object::SetHiddenValue()

Don't use v8::Object::SetHiddenValue() to keep a reference alive to the
buffer, we can just as easily do that from JS land and it's a lot faster
to boot.

Because the buffer is now a visible property of the write request
object, it's essential that we do *not* log it - we'd be effectively
serializing the whole buffer to a pretty-printed string.