Fix occasional cleartask failures

[potiuk]

The cleartask tests occasionally failed due to not consistent
sequence in which task clearing was performed.

The query did not have ordering and sometimes the tasks were
returned in different order than expected.

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[potiuk]

@uranusjr - here is another case where sorting will fix the occasional "flakiness" - i saw those test failing in at least 4-5 cases during last 24 hours. and if we sort it, they wont (the error was that clearing tasks failed because randomly a "dependent" task was cleared first before the task it depended on - so the tests failed).

If we sort it according to task_id (which guarantees that we will never clear a dependent task before the one it depends on) - those "clear" tests will always succeed. I think this is really simple solution to a difficult problme that we are struggling for weeks.

[potiuk]

@uranusjr - I've added comments to explain why order_by is equivalent to topological_sort - just to make sure this discussion is captured. But I really think trying to do it "right" is not the best approach, better to do it "simple".