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 apache/datafusion#543 (comment) 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