Core: Combat/threat system rewrite (PR #19930)

- PvE combat is now always mutual. UNIT_FLAG_IN_COMBAT is backed by actual references to the units we're in combat with.
- PvP combat is now also tracked, and almost always mutual; spells like Vanish and Feign Death can break this rule. That means we can easily determine a list of players we're fighting.
- By extension, IsInCombatWith now has sensible behavior when invoked on nonplayers.
- Threat and combat systems are no longer the same.
  - They still have an enforced relationship (threat implies combat - clearing combat clears threat)...
  - ...but we can have combat without threat. A creature (with threat list) isn't considered to be engaged until it has an entry on its threat list...
  - ...which means we can now faithfully replicate retail engage behavior. Combat on projectile launch - engagement start on projectile impact. Yay for progress!
- AI method refactor, as already ported in 6113b9d - `JustEngagedWith`, `JustEnteredCombat` and `JustExitedCombat`.
- Vehicle threat is now properly pooled on the main vehicle body (fixes #16542).
- Various edge case bug fixes for threat redirects (Misdirection "cancelling" Vigilance and similar).
- Target re-selection is now significantly faster.
- Fixed a ton of other smaller edge case bugs, probably.

Closes #7951 and #19998.

(cherry picked from commit 532ab1c7f8)

src/server/game/Grids/ObjectGridLoader.cpp

@@ -274,7 +274,7 @@ void ObjectGridStoper::Visit(CreatureMapType &m)
     {
         iter->GetSource()->RemoveAllDynObjects();
         iter->GetSource()->RemoveAllAreaTriggers();
-        if (iter->GetSource()->IsInCombat() || !iter->GetSource()->GetThreatManager().areThreatListsEmpty())
+        if (iter->GetSource()->IsInCombat())
         {
             iter->GetSource()->CombatStop();
             iter->GetSource()->GetThreatManager().ClearAllThreat();