[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

common/shared_tasks.cpp

@@ -7,7 +7,7 @@ std::vector<SharedTaskActivityStateEntry> SharedTask::GetActivityState() const
 	return m_shared_task_activity_state;
 }
 
-std::vector<SharedTaskMember> SharedTask::GetMembers() const
+const std::vector<SharedTaskMember>& SharedTask::GetMembers() const
 {
 	return m_members;
 }