Fix timestamp bugs in rtorrent config file and add a general method to display them

[chros73]

Bugs:

1. tm_completed value wasn't created after hash checked data (Fix timestamp bugs in rtorrent config file and add a general method to display them pyroscope/pyrocore#24)
2. last_active field gets overwritten after stopping/starting a torrent or restarting rtorrent. This bug was introduced here: pyroscope@e580adc (Fix timestamp bugs in rtorrent config file and add a general method to display them pyroscope/pyrocore#24)
3. Fix type of method d.last_active for future compatibility: it should be only simple instead of simple|value (Fix type of method d.last_active in rtorrent-0.8.6.rc for future compatibility pyroscope/pyrocore#18)

Enhancement:
4. replace d.last_active.print method with a general one that can be used to display all the timestamps (pyroscope#24)

Note: use the old 0.8.6 scripting style.