Added option for song timers behaviour. Fixes bug 8265

- Fixes bug 8265: http://hercules.ws/board/tracker/issue-8265-bragis-poem-overlapped-20-seconds-effect/
- Added setting in /conf/battle/skill.conf song_timer_reset: switch between official behaviour (songs don't reset time from prior songs) or eathena (every song resets the timer)