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/BattlegroundWS.cpp

@@ -62,10 +62,8 @@ BattlegroundWS::~BattlegroundWS()
 {
 }
 
-void BattlegroundWS::Update(uint32 diff)
+void BattlegroundWS::PostUpdateImpl(uint32 diff)
 {
-    Battleground::Update(diff);
-
     if (GetStatus() == STATUS_IN_PROGRESS)
     {
         if (GetStartTime() >= 25*MINUTE*IN_MILLISECONDS)
@@ -256,8 +254,7 @@ void BattlegroundWS::RespawnFlagAfterDrop(uint32 team)
 
     PlaySoundToAll(BG_WS_SOUND_FLAGS_RESPAWNED);
 
-    GameObject *obj = GetBgMap()->GetGameObject(GetDroppedFlagGUID(team));
-    if (obj)
+    if (GameObject *obj = GetBgMap()->GetGameObject(GetDroppedFlagGUID(team)))
         obj->Delete();
     else
         sLog->outError("unknown droped flag bg, guid: %u", GUID_LOPART(GetDroppedFlagGUID(team)));