Client dns packet id uses Random.int

[cfcs]

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.

[hannesm]

as noted in #31, there's as well weird random numbers in the server.