Optimize ReducePercentile method

[bsideup]

1. do not call len(allValues) twice
2. move sorting after index check

• CHANGELOG.md updated
• Rebased/mergable
• Tests pass
• Sign CLA (if not already signed)

[jwilder]

This code has changed quite a bit since #5196 landed. I believe this code is now located here: https://github.com/influxdata/influxdb/blob/master/influxql/call_iterator.go#L760-L798 This PR needs to be updated if it's still an issue.

@benbjohnson @jsternberg

[bsideup]

@jwilder it's still there, I will update

[bsideup]

@jwilder done. Please take a look :)

A little bit of explanation:

1. We shouldn't sort array until we decide to use it as a sorted (i.e. corner cases when i < 0 || i > len(a) )
2. We can reuse len(arr) call. Not that critical, but still saves some CPU cycles :)

[jsternberg]

👍

[jwilder]

👍