Scripts/VioletHold: rewrote the whole instance

thanks @MitchesD for help

(cherry picked from commit df21162fe44d2ff29c201a9004586f560789c38b)
(cherry picked from commit 3a4f54197c05ce4209c7e659acfd382896607265)
(cherry picked from commit f50df728201849ee7185d33c5a061c91e781c8de)
(cherry picked from commit 95bae7143f0f4bb7d5975f8a61daa3c533f48225)
(cherry picked from commit 5406f82044e7bd6ceb024a67f33c1674400cdc1f)

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

@@ -446,7 +446,13 @@ BossAI::BossAI(Creature* creature, uint32 bossId) : ScriptedAI(creature),
     instance(creature->GetInstanceScript()),
     summons(creature),
     _boundary(instance ? instance->GetBossBoundary(bossId) : NULL),
-    _bossId(bossId) { }
+    _bossId(bossId)
+{
+    scheduler.SetValidator([this]
+    {
+        return !me->HasUnitState(UNIT_STATE_CASTING);
+    });
+}
 
 void BossAI::_Reset()
 {
@@ -457,6 +463,7 @@ void BossAI::_Reset()
     me->ResetLootMode();
     events.Reset();
     summons.DespawnAll();
+    scheduler.CancelAll();
     if (instance)
         instance->SetBossState(_bossId, NOT_STARTED);
 }
@@ -465,15 +472,13 @@ void BossAI::_JustDied()
 {
     events.Reset();
     summons.DespawnAll();
+    scheduler.CancelAll();
     if (instance)
         instance->SetBossState(_bossId, DONE);
 }
 
 void BossAI::_EnterCombat()
 {
-    me->SetCombatPulseDelay(5);
-    me->setActive(true);
-    DoZoneInCombat();
     if (instance)
     {
         // bosses do not respawn, check only on enter combat
@@ -484,6 +489,11 @@ void BossAI::_EnterCombat()
         }
         instance->SetBossState(_bossId, IN_PROGRESS);
     }
+
+    me->SetCombatPulseDelay(5);
+    me->setActive(true);
+    DoZoneInCombat();
+    ScheduleTasks();
 }
 
 void BossAI::TeleportCheaters()