stream: read refactor

This slightly refactors read by moving side effects out of howMuchToRead.

We don't actually have to set state.needReadable = true; in howMuchToRead since read handles 0 return as needReadable.

 streams/readable-bigunevenread.js n=1000          *      8.80 %       ±7.47% ±9.94% ±12.94%

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines