Updates the arena rating system to use Matchmaker Rating.

The arena_team_stats sql in this commit sets all ratings to 0.
You start with 0 Team Rating and 1500 Matchmaker Rating.
Personal rating is moved to character_arena_stats, and will not reset when disbanding an arena team.
This new system is based on all the info I could get, but Blizzard didn't leak much info about the new system.
At least this system is really close to the official system.
Any issues found should be reported in an issue.
Huge thanks go to Aokromes for the intensive testing he has done with me.
Also thanks to Parzival for hearing me out and giving his ideas.
Fixes issue #3189

--HG--
branch : trunk

src/server/game/Battlegrounds/Battleground.h

@@ -513,6 +513,8 @@ class Battleground
         uint32 GetArenaTeamIdForTeam(uint32 Team) const             { return m_ArenaTeamIds[GetTeamIndexByTeamId(Team)]; }
         void SetArenaTeamRatingChangeForTeam(uint32 Team, int32 RatingChange) { m_ArenaTeamRatingChanges[GetTeamIndexByTeamId(Team)] = RatingChange; }
         int32 GetArenaTeamRatingChangeForTeam(uint32 Team) const    { return m_ArenaTeamRatingChanges[GetTeamIndexByTeamId(Team)]; }
+        void SetArenaMatchmakerRating(uint32 Team, uint32 MMR){ m_ArenaTeamMMR[GetTeamIndexByTeamId(Team)] = MMR; }
+        uint32 GetArenaMatchmakerRating(uint32 Team)                { return m_ArenaTeamMMR[GetTeamIndexByTeamId(Team)]; }
         void CheckArenaWinConditions();
         void UpdateArenaWorldState();
 
@@ -649,6 +651,7 @@ class Battleground
         uint32 m_ArenaTeamIds[BG_TEAMS_COUNT];
 
         int32 m_ArenaTeamRatingChanges[BG_TEAMS_COUNT];
+        uint32 m_ArenaTeamMMR[BG_TEAMS_COUNT];
 
         /* Limits */
         uint32 m_LevelMin;