[Shared Tasks] Avoid erasing shared tasks while iterating (#2348)

This wasn't safe since the erase would invalidate iterators used
internally by the range loops.

Shared tasks with no members are now also cleaned up

Make GetMembers return reference instead of copy (this is used a lot)

Add rule for shared task terminate time for easier debugging

world/shared_task_manager.h

@@ -132,7 +132,6 @@ protected:
 	void RecordSharedTaskCompletion(SharedTask *s);
 	void RemoveAllMembersFromDynamicZones(SharedTask *s);
 	void StartTerminateTimer(SharedTask* s);
-	void Terminate(SharedTask* s);
 
 	// memory search
 	std::vector<SharedTaskMember> FindCharactersInSharedTasks(const std::vector<uint32_t> &find_characters);