Core/Combat: reset the ThreatManager update interval upon engaging thee creature and move ThreatClear packet sending into the update cycle to mimic retail behavior

[Ovahlord]

Changes proposed:

• the threatmanager will now continue to update even when dead to support sending pending packets when dead
• TheatClear is now put behind the threat update interval to mimic retail
• This fixes an inconsistency in the client where HighestThreatUpdate and ClearTheat would be sent at the same time, resulting in the creature retaining the threat display, causing the client to glitch out and being stuck in this red'ish state

Tests performed:

• tested ingame

Known issues and TODO list: (add/remove lines as needed)

• This treatment should also be applied for the CombatManager as the combat and threat update but should happen at the same time.