When query result reach limit, the Coordinator in FE will send a cancel
request to BE to cancel the query. And when being cancelled, BE will report
query status to FE for debug purpose. But actually it is not necessary
and will generate too many logs.
So I add a CancelReason to distinguish the difference between 'normally'
cancellation and 'internal error' cancellation. if 'normally' cancelled,
no status will be reported from BE.
When query reach limit, or user cancel it actively, it is being cancelled 'normally'.
Otherwise, the query is cancelled due to internal error, which will need
a report from BE.
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