Questlog fixes

- Improved memory usage of the quest log system. (saves up to 75kB per
  online character). Fixes issue #133.
- Fixed various issues with quest entries disappearing from characters
  without an apparent reason, or monster kill counters getting stuck -
  the issues were caused by a de-synchronization between the two
  parallel questlog arrays in map_session_data.
- Added some code documentation.
- Thanks to Ind.

Signed-off-by: Haru <haru@dotalux.com>