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.

src/server/game/AI/ScriptedAI/ScriptedCreature.cpp

@@ -520,11 +520,12 @@ void BossAI::TeleportCheaters()
     float x, y, z;
     me->GetPosition(x, y, z);
 
-    ThreatContainer::StorageType threatList = me->GetThreatManager().getThreatList();
-    for (ThreatContainer::StorageType::const_iterator itr = threatList.begin(); itr != threatList.end(); ++itr)
-        if (Unit* target = (*itr)->getTarget())
-            if (target->GetTypeId() == TYPEID_PLAYER && !CheckBoundary(target))
-                target->NearTeleportTo(x, y, z, 0);
+    for (auto const& pair : me->GetCombatManager().GetPvECombatRefs())
+    {
+        Unit* target = pair.second->GetOther(me);
+        if (target->IsControlledByPlayer() && !CheckBoundary(target))
+            target->NearTeleportTo(x, y, z, 0);
+    }
 }
 
 void BossAI::JustSummoned(Creature* summon)