ntp: use previous root delay/disp in interleaved mode

ntp: use previous root delay/disp in interleaved mode

When calculating the root delay and dispersion of a sample measured in
the interleaved mode, use the root delay and dispersion values from
the previous response (to which the TX timestamp corresponds). If the TX
timestamp is combined with the RX timestamp of the latest response (e.g.
in the symmetric mode), use the maximum of the previous and latest root
delay/dispersion.