updated min-tso query hang and tiflash crash

[fzhedu]

Bug Report

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1. Minimal reproduce step (Required)

after #4307, scheduling new tasks only after releasing threads by threads, there comes a bug: after deleting a finished query, the min-tso query would be updated, if the updated min-tso query has waiting tasks, and no threads is released possiblely due to resouces deadlock, then the updated min-tso query would hang.

on the other hand, when the finish processing of this task schedules new tasks, the possible exceptions should be thrown to the to-be-scheduled query, rather than the current task. If this happen, the destruction of the current task would terminate the tiflash process.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiFlash version? (Required)

master