Core/Units: Power handling improvements

* Don't include creature_template power multipliers in CreateMana updatefield
* Allow NPCs to have power types other than mana
* Add missing Essence config regeneration rate
* Fixed demon hunter powers not decaying immediately after combat
* Fixed some powers decaying immediately out of combat after energizing (for example holy power)
* Replace hardcoded list of powers to set to full on levelup with a db2 flag check
* Updated Creature::GetPowerIndex for 10.1 new power types
Port From (https://github.com/TrinityCore/TrinityCore/commit/1325e0c4b279d590acbcbb05d6abdf5fdb66a882)

Source/Game/Entities/Creature/CreatureData.cs

@@ -271,16 +271,6 @@ namespace Game.Entities
         public uint BaseMana;
         public uint AttackPower;
         public uint RangedAttackPower;
-
-        // Helpers
-        public uint GenerateMana(CreatureTemplate info)
-        {
-            // Mana can be 0.
-            if (BaseMana == 0)
-                return 0;
-
-            return (uint)Math.Ceiling(BaseMana * info.ModMana * info.ModManaExtra);
-        }
     }
 
     public class CreatureLocale