[9918] Fixed player's tapped creature loot access by group in diff cases

* If player tap creature in group and leave then group will have access to creature loot if not disbanded
* If player tap creature and after join to group then creature loot will accesable only by player
* Also RewardPlayerAndGroupAtKill divided to simgle player and group reward versions used for group tap
  and single player tap cases.

src/game/ObjectGuid.h

@@ -130,6 +130,7 @@ class MANGOS_DLL_SPEC ObjectGuid
         PackedGuidReader ReadAsPacked() { return PackedGuidReader(*this); }
 
         void Set(uint64 const& guid) { m_guid = guid; }
+        void Clear() { m_guid = 0; }
 
         // Possible removed in future for more strict control type conversions
         void operator= (uint64 const& guid) { m_guid = guid; }