Client dns packet id uses Random.int

I took a shortcut here: https://github.com/roburio/udns/blob/master/client/udns_client.ml#L20

Leaving this issue here so we don't forget about it.

EDIT: Also it's off by one (should be `0x10000`):
```
Random.int bound returns a random integer between 0 (inclusive)
and bound (exclusive). bound must be greater than 0 and less than 2^30.
```