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/BattlegroundSA.h

@@ -410,12 +410,8 @@ struct BG_SA_RoundScore
 /// Class for manage Strand of Ancient battleground
 class BattlegroundSA : public Battleground
 {
-    friend class BattlegroundMgr;
-
     public:
-        /// Constructor
         BattlegroundSA();
-        /// Destructor
         ~BattlegroundSA();
 
         /**
@@ -423,7 +419,7 @@ class BattlegroundSA : public Battleground
          * -Update timer
          * -Round switch
          */
-        void Update(uint32 diff);
+        void PostUpdateImpl(uint32 diff);
 
         /* inherited from BattlegroundClass */
         /// Called when a player join battle