reference: randomize reference time

reference: randomize reference time

In unauthenticated interleaved symmetric NTP mode we should be now
careful with the reference timestamp as it may be useful with the peer
delay for estimating the local receive timestamp and increasing the
chance of spoofing a valid response from the peer.

When updating the reference time, add a random error of up to one second
to make it less sensitive when disclosed to NTP and cmdmon clients.