Add performance tuning

Add a diff with and without player in server. The server will tune bot activity to reach the desired server tick speed (in ms), as what CMaNGOS' playerbots are using.

ce4bd79d2d is used as a reference.

src/PlayerbotAIConfig.h

@@ -178,6 +178,10 @@ class PlayerbotAIConfig
         uint32 playerbotsXPrate;
         uint32 botActiveAlone;
 
+        uint32 enablePrototypePerformanceDiff;
+        uint32 diffWithPlayer;
+        uint32 diffEmpty;
+
         bool freeMethodLoot;
         int32 lootRollLevel;
         std::string autoPickReward;