proto: add a callback_data argument to sync and ping

In both cases the initial data is returned as-is by the respective other side. A uint32 is enough to use it as key in a hashmap or something though where that value is needed both sides need to do due dilligence that the value is correct (i.e. not easily guessable).

We don't need it in libei, so we can just send an increasing number down (we could send 0 but that could mean code always expects zero).