Ensure no concurrent read

Currently _from will be called multiple times concurrently if you do a something asynchronously read

``` js
var rs = from(function(size, cb) {
  console.log('reading')
  setTimeout(function() {
    console.log('not reading')
    cb(null, 'test')
  }, 100)
})

rs.resume()
```

Running the above will print something like

```
reading
not reading
reading
reading
```

This PR fixes this