luci-app-statistics: modify default amount of data items in RRD

Increase the default number of data items in the RRD database
from 100 to 144. That leads to better matching summarising/averaging
moments between day & week and week & month at the averaging intervals:
30sec, 10min, 70 min, 5h10min, 2d13h

Previous 100 led too easily to situations, where the longer period's
more scarce data gets selected for displaying in the graph. That could
happen if the longer period's last data point was stored more recently
than the last data item in the originally required period.
(E.g. if the last "week data item" was more recent than the last
"day data item", the week data was used for the day chart.)

(Note: this change only applies in a live router if the RRD database is
empty. E.g after reboot or after emptying the RRD database dir.)

Reference to discussion at #4065

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>