Rewrote the hard monster AI. Monsters will now behave a lot closer to of...

...ficial servers:

* Monsters will now attack immediately when they are chasing a target and it comes into attack range (bugreport:7370)
* Monsters will now chase their target during their aDelay, but they still have to wait for aMotion to be able to move again (bugreport:9269)
* Monsters will now rethink their chase in a configurable interval (see monster_chase_refresh in monster.conf), official value is once per cell, previously it was once per 3 cells
* Monsters will now stop when they rethink their chase and their target is gone (player hides or target loot was picked), regardless of the monster_ai setting (note: if you want the old, stupid behavior, just increase monster_chase_refresh instead)