Feature Request

Is your feature request related to a problem? Please describe:

Described in tiflash issue: pingcap/tiflash#4631
Describe the feature you'd like:

1. add two system variable: tiflash_fine_grained_shuffle_stream_cout and tiflash_fine_grained_shuffle_batch_size
2. add tag in MPPTask to indicate whether enable fine grained shuffle or not.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: