Core/Game: Rewrote the ScriptMgr to support script reloading.

* Finally this commit enables dynamic script hotswapping
  and finished the PR #15671.
* Split the storage layout to use optimized storages
  for database bound and unbound scripts.
* Add several unload workers to reload scripts correctly
  -> Requires further investigation.
* Fixes memory leaks in ScriptMgr when dropping invalid scripts.
* Fixes VehicleScripts
* Makes OutdoorPvP scripts reloadable
* Makes InstanceMapScripts reloadable
* Makes CommandScripts reloadable

(cherry picked from commit 9cc97f226d)

src/server/game/Spells/SpellScript.cpp

@@ -16,6 +16,7 @@
  */
 
 #include "Spell.h"
+#include "ScriptMgr.h"
 #include "SpellAuras.h"
 #include "SpellScript.h"
 #include "SpellMgr.h"
@@ -50,6 +51,12 @@ void _SpellScript::_Init(std::string const* scriptname, uint32 spellId)
     m_currentScriptState = SPELL_SCRIPT_STATE_NONE;
     m_scriptName = scriptname;
     m_scriptSpellId = spellId;
+
+#ifdef TRINITY_API_USE_DYNAMIC_LINKING
+    // Acquire a strong reference to the binary code
+    // to keep it loaded until all spells are destroyed.
+    m_moduleReference = sScriptMgr->AcquireModuleReferenceOfScriptName(*scriptname);
+#endif // #ifndef TRINITY_API_USE_DYNAMIC_LINKING
 }
 
 std::string const* _SpellScript::_GetScriptName() const