ntp: refresh IP addresses periodically

ntp: refresh IP addresses periodically

Refresh NTP sources specified by hostname periodically (every 2 weeks
by default) to avoid long-running instances using a server which is no
longer intended for service, even if it is still responding correctly
and would not be replaced as unreachable, and help redistributing load
in large pools like pool.ntp.org. Only one source is refreshed at a time
to not interrupt clock updates if there are multiple selectable servers.

The refresh directive configures the interval. A value of 0 disables
the periodic refreshment.

Suggested-by: Ask Bjørn Hansen <ask@develooper.com>