We should delete a job by directly talking to the allocated
task and telling it to shutdown. Today we shut down a job
via the persistent task framework. This is not ideal because,
while the job has been removed from the persistent task
CS, the allocated task continues to live until it gets the
shutdown message.
This means a user can delete a job, immediately delete
the rollup index, and then see new documents appear in
the just-deleted index. This happens because the indexer
in the allocated task is still running and indexes a few
more documents before getting the shutdown command.
In this PR, the transport action is changed to a TransportTasksAction,
and we invoke onCancelled() directly on the matching job.
The race condition still exists after this PR (albeit less likely),
but this was a precursor to fixing the issue and a self-contained
chunk of code. A second PR will followup to fix the race itself.
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