fix: only flush the queue after open if not already writing

[nlf]

this impacted code that followed a pattern like the following:

const s = fsm.createWriteStream(p)
s.on('open', () => {
  s.write('hello ')
  s.write('human')
  s.end()
})

since emitting the open event is synchronous and calls all of its handlers synchronously, the first write starts immediately and the second write is queued, then the _onopen method calls _flush() which races the _onwrite call from the first write. by only calling _flush if we're not already writing, we avoid this situation where flush gets called twice and we prevent lost data.

for npm/statusboard#631
closes #24