be more resilient to ntp adjustments

[Nieuwejaar]

Because we start the lldpd daemon before external networking is up, we start collecting neighbor information prior to ntp running. This can lead to clearly bogus results like:

root@oxz_switch1:~# lldpadm ne
On interface qsfp0/0
        first seen:   13723d14h40m12s ago
        last seen:         1s ago
        last changed: 13723d14h40m12s ago
        ChassisID: MacAddress(MacAddr { a: [68, 76, 168, 46, 202, 13] })
        PortId: InterfaceName("Ethernet25/1")
        TTL:  120 seconds
        PortDescription: london-dac
        System Name: eveningstar