Further TaskGroup improvements for fail fast

[amorton]

More work from #1672

From review of the PR #2193

• the TaskGroup does not order tasks by position(), they are ordered on insertion order and it just happens to be that. Change to guarantee insertion order.
• the check in TaskGroup. shouldFailFast() is not strict enough, we want to check for tasks before the current task that means we should check the position not reference quality.
• some comment and code cleanup.