wrong singleflight implementation for stats' syncload

[winoros]

Bug Report

We use the setWorking for sync load's duplicate task detection.

It will return a boolean value directly after it finds duplicates.

And we can see that the writeToResultChan also returns directly by writing an ok result to the resultCh.

So the duplicated task will directly return without waiting for the executing one to finish.

As a result, if there are two TiDB nodes and we run sysbench's select_random_points with more than 2 threads, we will find out the following slow query:

The sync load doesn't timeout but we still have the SQLs executed without stats.
Such a slow query would disappear if the thread number is equal to or less than 2.