[9597] create battleground map at bg creation

[newsbot]

<pre>m src/game/BattleGround.cpp
m src/game/BattleGround.h
m src/game/BattleGroundMgr.cpp
m src/game/MapInstanced.cpp
m src/game/MapInstanced.h
m src/game/MapManager.cpp
m src/game/MapManager.h
m src/shared/revision_nr.h

[9597] create battleground map at bg creation

when a bg gets created it will also get it's map
(old behaviour was, that map got's created when first player entered the instance)

the reason why battlegroundmaps aren't instantiated with a player object:
* the only information from player-class we need is player->GetBattleGround()
also we can't use anything else
(e.g. playerlevel can be outside of bg-levelrange)
-> cause bgs depend pretty much on their maps this caused circualr dependencies between map,bg,player
* battlegroundmaps will _always_ be prepared by the bgsystem to set the proper data
* there is not much shared in the creational process with dungeonmaps
even the functioncall is different since players need to enqueue first for bgs,
therefore again the player's way to that instance is only through the bgMgr</pre>



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