[Core]Proximity aggro and linking mechanics

[Zarant]

Comparing to the classic client, aggro and linking mechanics feels off. Old TBC videos only offers circunstantial evidence, there is no reliable way to check if that behavior was changed during TBC, so I'm gonna post the research I did.

How it's supposed to work:

1. Hostile NPCs should check if an attackable unit is nearby every 1.5s
2. Once a NPC enters combat, it should keep doing an aggro pulse every 1.5s
3. When a creature is in combat, the periodic aggro pulses will keep happening but this time will look for nearby friendly units of the same faction to assist it. This assist mechanic will only call upon mobs that are out of combat.
4. Inside instances, NPCs will keep checking for hostile units nearby during their periodic aggro pulse, regardless if they are on evade mode or already in combat.
5. In open world, creatures won't do periodic aggro pulses if they are on evade mode.
6. If a NPC sucessfully manage to call another creature to assist, both creatures should be linked, meaning that any action that puts you on the threat list of one mob should immediately put you on the threat list of all other mobs linked.
7. Any hostile actions towards any given mob should refresh the leash timer on all other linked mobs

Research: (1) I did the same test several times and figured out that npc aggro can happen anywhere between 0 to 1.5s: https://youtu.be/NbWMmAE0Hi0

(2,3,6,7) Creature assist always happens 1.5s after the first NPC is pulled, using a hostile ability will refresh the leash timer on all linked mobs: https://youtu.be/T8Qq4-6b2DM

(6,7) Pulling 2 mobs at the exact same time will prevent the assist mechanic from happening allowing you to split pull mobs of the same faction: https://streamable.com/msb61g

(4) Last boss in Steamvaults is pulled and the mage enters combat only when he gets close enough to it https://www.youtube.com/watch?v=CS2ZRWzujpo

(4,5) Mobs on evade mode in DM north can aggro players on their way back: https://youtu.be/hA8eMj8KwzY?t=1094

[Godmatik]

Rev 4020

Linking/Proximity improvements still need to be implemented