[NF] Delete old traffic_daily entries that are no longer needed

This also introduces a new application.ini parameter:

; the traffic_daily table can get pretty full and most of the long term information
; are in the MRTG / other stats files anyway. If you want to keep this data in the
; database, set the following to false. If it is true, when the daily task runs
; to populate this table, it will also delete any entries older than
; cli.traffic_differentials.stddev_calc_length days (this parameter is set above).
Merge branch 'nf-delete-daily-traffic'
Update changelog