[chrony-users] Add configuration option to disable randomized transmit timestamp

[egg car]

In the scenario of local datacenter, we've been monitoring client system
time quality by logging transmit timestamp in clients ntp requests on NTP
server.
But this seems not working correctly when we're using chrony instead of
ntpd on some machines.
I know this is a default 'feature' of chrony, but should we have a switch
to turn off this behavior?