stream: micro-optimize high water mark calculation

Don't iterate over all 32 bits, use some hacker's delight bit twiddling
to compute the next power of two.

The logic can be reduced to `n = 1 << 32 - Math.clz32(n)` but then it
can't easily be backported to v2.x; Math.clz32() was added in V8 4.3.

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>