Implement fast path for QueryStageExec when writing 1 shuffle partition

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do. As mentioned in https://github.com/apache/arrow-datafusion/pull/543#discussion_r650883972 we could optimize for the case where there is 1 output partition.

Describe the solution you'd like Avoid the cost of computing hashes when output partition count is 1.

Describe alternatives you've considered None

Additional context None