there are some memory leak in TTL's implement

[lcwangchao]

If you run tidb with a very long time with many TTL tables, you can seem some memory leak:

1. dettachStatsCollector , see:

tidb/session/session.go

Lines 3660 to 3665 in 9e20208

	func detachStatsCollector(s *session) *session {
	s.statsCollector = nil
	s.idxUsageCollector = nil
	return s
	}

it only sets two collectors to nil but did not call Delete for it.

2. The context created here is not guaranteed to be cancelled finally:

tidb/ttl/ttlworker/task_manager.go

Lines 391 to 399 in 9e20208

	ctx, cancel := context.WithCancel(m.ctx)
	scanTask := &ttlScanTask{
	ctx: ctx,
	TTLTask: task,
	tbl: table,
	statistics: &ttlStatistics{},
	}