Core/Battleground: Some (minor) code optimizations

- All access to Battleground vars are done using funtions (Remove friendship from Battleground and BattlegroundMgr)
- Move specific 47 mins arena check to battleground code as its generic to all arenas
- Create PreUpdateImpl and PostUpdateImpl functions. Specific battlegrounds will implement those if needed (No more need of call Battleground::Update inside specific BGs)
- Modify Create function to use only one parameter
- Remove EventPlayerCapturedFlag from Battleground class, only one BG implements it and it's not used outside that BG.

src/server/game/Battlegrounds/Zones/BattlegroundAA.h

@@ -30,12 +30,9 @@ class BattlegroundAAScore : public BattlegroundScore
 
 class BattlegroundAA : public Battleground
 {
-    friend class BattlegroundMgr;
-
     public:
         BattlegroundAA();
         ~BattlegroundAA();
-        void Update(uint32 diff);
 
         /* inherited from BattlegroundClass */
         virtual void AddPlayer(Player *plr);