Core/Battlegrounds: Implement power resets on arena-fight preparations (first 60 seconds before arena-fight kicks off) (Thanks to Kaelima)

Restores all player powers every 5 seconds untill arena starts (fixes possible exploit with entering arena battle with full rage or runic power)
(The 5 second timer is not 100% confirmed, as official pages only states that the power is reset after prep-time - needs feedback)
Closes issue 4805

--HG--
branch : trunk

src/server/game/Battlegrounds/Zones/BattlegroundSA.cpp

@@ -474,8 +474,7 @@ void BattlegroundSA::TeleportPlayers()
                 plr->SpawnCorpseBones();
             }
 
-            plr->SetFullHealth();
-            plr->SetPower(POWER_MANA, plr->GetMaxPower(POWER_MANA));
+            plr->ResetAllPowers();
             plr->CombatStopWithPets(true);
 
             if (plr->GetTeamId() == attackers)