50% performance increase in ReadStream by removing one bind.

Bind is dangerous for performance. Ideally we should not use bind with iterators, as we can lose a lot of performance.

In this benchmark https://github.com/mcollina/node-levelgraph/blob/master/benchmarks/readStreamLeveldb.js the attached patch gives a 50% increase in throughput

Before:
```
$ node benchmarks/readStreamLeveldb.js
total time 2346
reads/s 42625.745950554134
```

After:
```
$ node benchmarks/readStreamLeveldb.js
total time 1614
reads/s 61957.868649318465
```