Enhancement

https://github.com/pingcap/tiflash/blob/master/dbms/src/DataStreams/ParallelInputsProcessor.h#L287-L289
https://github.com/pingcap/tiflash/blob/master/dbms/src/DataStreams/ParallelInputsProcessor.h#L314-L315

If the number of threads is 10 and the number of streams is 10.
Only one stream A can be read 100 times and the other streams can only be read once.
In the end, only one thread will execute stream A, instead of 10 threads executing together.