Client dns packet id uses Random.int

[hannesm]

from robur-coop/udns#9

@cfcs reports:
I took a shortcut here: https://github.com/mirage/ocaml-dns/blob/master/client/dns_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 further reports:
Server code (and resolver) should also unify the use of random, maybe sth in dns.mli should be around that takes a rng and produces a proper id.

[hannesm]

this has been resolved by #184 #185