stream: rename poorly named function

roundUpToNextPowerOf2() does more than just rounding up to the next
power of two.  Rename it to computeNewHighWaterMark().

PR-URL: https://github.com/nodejs/node/pull/2479


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>