[WIP] Improved performance of journal cleaning

Refs #21

**Performance testing**
- Redis 2.8
- 10.000 keys
- 10 tags each
- cleaning by 5 tags

**PHP**
- current - 9.5 seconds on average
- after update - 0.25 seconds on average

**Lua**
- 0.45 seconds on average - no meassurable improvement (yet)

**disadvantage**
- deleted keys could remain in priority sets or in tag sets (increase of memory usage)