Dynamic Creature/Go spawning:

- True blizzlike creature spawn/respawn behavior - new creature = new object
 - Toggleable spawn groups (with C++/SAI/command options to use them)
 - Custom feature: dynamic spawn rate scaling. Accelerates respawn rate based on players in the zone.
 - Backward compatibility mode (set via group and for summons)
   to support creatures/gos that currently don't work well with this
   (this should be removed once the exceptions are fixed)

Fixes and closes #2858
Tags #8661 as fixable.
Fixes and closes #13787
Fixes #15222.

(cherry picked from commit 59db2eeea0a35028779fd76372ae06cc98c8086f)

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

@@ -462,7 +462,7 @@ void BossAI::_Reset()
     events.Reset();
     summons.DespawnAll();
     scheduler.CancelAll();
-    if (instance)
+    if (instance && instance->GetBossState(_bossId) != DONE)
         instance->SetBossState(_bossId, NOT_STARTED);
 }
 
@@ -548,12 +548,12 @@ bool BossAI::CanAIAttack(Unit const* target) const
     return CheckBoundary(target);
 }
 
-void BossAI::_DespawnAtEvade(uint32 delayToRespawn /*= 30*/, Creature* who /*= nullptr*/)
+void BossAI::_DespawnAtEvade(Seconds delayToRespawn, Creature* who)
 {
-    if (delayToRespawn < 2)
+    if (delayToRespawn < Seconds(2))
     {
-        TC_LOG_ERROR("scripts", "_DespawnAtEvade called with delay of %u seconds, defaulting to 2.", delayToRespawn);
-        delayToRespawn = 2;
+        TC_LOG_ERROR("scripts", "_DespawnAtEvade called with delay of " SI64FMTD " seconds, defaulting to 2.", delayToRespawn.count());
+        delayToRespawn = Seconds(2);
     }
 
     if (!who)