stream: cleanup pipeline destroyer

Slightly cleans up the destroyer logic. Make readable and writable use similar logic.

i.e.

    if (!err && reading && !writing && stream.writable) {
      return callback();
    }

    if (!err && !reading && writing && stream.readable) {
      return callback();
    }

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