ntpd: do not invalidate datapoints after step

Used to set p->filter_datapoint[i].d_dispersion = MAXDISP
and clear reachable bits, but this proved to be too agressive:
after step (tested with suspinding laptop for ~30 secs),
this caused all previous data to be considered invalid,
making us needing to collect full ~8 datapoins per peer
after step in order to start trusting them.
In turn, this was making poll interval decrease even after
step was done. (Poll interval decreases already before step
in this scenario, because we see large offsets and end up with
no good peer to select).

function                                             old     new   delta
reset_peer_stats                                     157     139     -18

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>