Faster iterator

Hey, I've create a small module that will run something in `process.nextTick` until it's been done `process.maxTickDepth` number of times, when it'll instead run it in a `setImmediate` - so you'll get the performance of process.nextTick while still working as expected.

running the stream benchmarks (from levelup) shows some really fantastic results using this PR.

```
Filled source! Took 3135ms, streaming to destination...
Done! Took 10389ms, 331% of batch time
 node-levelup (master) node test/benchmarks/stream-bench.js                                 0:05:22
--------------------------------------------------------------
Filled source! Took 3004ms, streaming to destination...
Done! Took 3529ms, 117% of batch time
```

Also, kudos to @mafintosh who thought about this approach to begin with.