Core/Game: Include cleanup

* Mostly aimed at removing Log/DatabaseEnv includes from other headers
* Fix most packet headers including other packet headers - moved common structures such as ItemInstance to their own files
* Moved SAI function definitions to source files (massive or requiring many different dependencies)

src/server/game/Handlers/BattleGroundHandler.cpp

@@ -26,6 +26,7 @@
 #include "ArenaTeam.h"
 #include "BattlegroundMgr.h"
 #include "Battleground.h"
+#include "BattlegroundPackets.h"
 #include "Chat.h"
 #include "Language.h"
 #include "Log.h"